gas peddel can be done after it's all in.I use a tierap/wiretie,smallest I can find about 1/16 wide to hold the cable on. also with the long arm it is helpful if you relocate the bracket on the trans so it pulls straighter and dosent put the cable in a bind as much as the std location,I made an extra bracket then the next time I had it appart I just sloted the lower hole in the irs bracket,possiably both holes I dont rember exzactly. and the new plastic bowden tubes are real nice they are much easyer on the cable, no steel on steel rubbing each time you depress the clutch.
After rereading this post, I have to ask what type hydraulic fluid was reccomended for the master cylinder and what was reccomended for the slave cylinder.... Anyone from the '50s / '60s era will have not so fond memorys of British autos requiring their own vegetable based hydraulic fluid... Use Dot fluid and the seals would swell up and the system shut down... Use of British fluid in US made cars allowed the seals to start leaking, and would not develop the pressure the system normally required....
I thought this might be a similar chemistery problem... 
Couldn't remember the name "GIRLING"....
how could you forget a name like that.with a car like yours, great for girl....ing
I remember that stuff in my Austin Healey. There was, like, two places on New England that sold it. Really popular stuff...not!
Lane! Took a look at your build pictures and think you have the gas pedal, rather than the trusty roller as Wild Bill has. I just usually flip the gas pedal to the rear as far as it can go, (you may be able to release the return spring to keep it there easily) then flip the clutch cable loop over the pedal hook, then go to the rear and attach a rubber band (think big, here, like a model airplane rubber band) to the rear of the cable using the giant wing nut on the rear adjustment stud as an anchor, then pull the rubber band back and attach it to something to give the cable a good (firm, not super tight) pull. A medium bungee cord would work well here, too.
Heading back to the cockpit, firmly grasp the pedal cluster and guide it into the hole in the tunnel and onto the pedal stop on the floor. The clutch cable will pull in and stay nicely in place without any rubber band or tie wrap around the pedal hook (I always worry about that tie wrap limiting motion during use after assembly, but what do I know?) and the tension from the rear actually makes the cluster feel more stable.
While guiding the cluster in place you can easily use both hands to set the brake pedal return spring AND the gas pedal return spring, and then install the mounting bolts (that, for me, is usually the harder part - they never seem to want to find the threads). Once all that is done, go ahead and install your brake MC piston shaft (you might want to leave the cluster mounting bolts a tad loose until you get that on), re-attach the gas cable, tighten the cluster bolts and the front is done.
I often just leave the rubber band/bungee in place at the back to keep tension on the cable while I mess around with the Bowden tube or other stuff, removing the band only when I can hold the cable with one hand while guiding the end through the throw-out arm and get the wing nut back on. Don't forget to grease the hell out of the entire length of the cable, especially where it goes through the Bowden tube. I think that I used a tranny bowden tube bracket from a bus on Pearl to get the cable angle right, but honestly can't remember. I'm sure someone on Shop Talk would have that answer in milliseconds.
BTW: really early Bowden tubes (before the fancy-shmancy teflon-lined ones came out) had a grease fitting on them to let you grease them every year ("Old Yeller" would love that) and I think the real 356's had fittings on the clutch tube, too, along with the pedal cluster (you DID add a fitting to the cluster when you had it out, like TC and I and Old Yeller recommended in the past, right???). 
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On my pedals you have to put the brake MC pushrod on before setting the pedal return spring. The job would have been somewhat easier if I had unhooked the gas pedal return spring (doh!).
I made sure the the clutch cable entered the steel tube in the tunnel in the center. Initially it was up against the side, which might have been what caused some of the problems with the previous cable, initiating the switch to hydraulic. I greased the poop out of the cable along the entire length, including pumping the bowden tube full of grease before installation. I have to say it works more smoothly now than it has for some time even with the hydraulic setup. The MC was probably failing slowly before it hit the point of no return. The deterioration was gradual enough that I didn't notice until shortly before it failed. It is smoother now than I ever remember it being with the cable before.
I have to say that the removable bowden tube bracket on the IRS tranny is a MUCH better setup than the cast-on piece on the swing axle one. I had to cut that off to put on the slave cylinder bracket, so I used a IRS bracket this time. Getting the cable through the tube, and then the tube in place is sooooooooooooooooooooo much easier this way.
and there are diferent length bowden tubes too justincase you need something a bit different, as for all the greezzee in the tube, thats good, but Ive seen some tubes full of grit&dirt, always blow them out befor greezing them, some breakleen would also help but be sure you dont splater that carap everywhere. I agree the vw peddels are a pain, and when your a big guy even more so then add some age to the big guy and ......hmm is it time for an automatic?
EPILOGUE: I got curious enough this morning to disassemble the master cylinder and confirmed my theory. On one side there is obvious wear on the inner wall of the cylinder, while one of the rubber seals shows severe wear on the same side. This was almost certainly caused by the fact that it was not possible to line up the pushrod so that it followed the axis of the cylinder. The slight but visible offset applied asymmetrical pressure that eventually caused it to fail.
I can't fault the SACO design as I used it in a non-standard and untested configuration. Right now I am happy with the cable setup, although it does take a bit more effort than the hydraulic. With the minor mods I made, some due to the previous switch to hydraulic, I think this will serve me just fine. If not, there's always the way Carey does his hydraulic installations.
I like your diognossis,thats what I would of thought or rong fluid. Somany guys just think things can be stuck on and work just fine.well this is a prima exzample of what happens. You have to be smarter that what your worken on. the angle of the dangle needs to be right nomatter what your worken on. Breaks, clutch,rockerarms,clutch cable ,it all needs to be refined to work at it's best.and thats not always easy,and not just bolt on.
Im gladd you took it appart to see &determan what the issue was. Somany would just replace it & go on...then cuss the junk parts when that one died, when it wasent tha parts fault at all.
Lane,
i came to the same conclusion as you, when I was messing with the clutch system. I went back to the cable also. The one thing I didn't do that I should have was to change to the later super beetle type of t.o. Brg and clutch arm. Did you make those upgrades?
Greg.
PS.
loving my Abarth. It's the coolest pocket rocket toy car I've ever owned.
Haven't changed the throughout or anything major like that. So far, so good on the cable.
You gonna make it to Carlisle this year, Greg?
Lane,
Going to try!
Greg