Hey, that's pretty trick.
I wasn't clear on your problem description - did the PO weld the throttle cable to the transport tube inside of the tunnel and now it won't move?
Or...is the transport tube welded to the inside of the tunnel AND you could snake a new cable through it?
Or....Did you just decide to upgrade to a better cable-transport for the hell of it?
Either way, if you decide to fully use the new system, it might be more difficult to run the new cable sheath through the tunnel. Not impossible, just harder, BUT! Once you get to the rear of the cockpit you have to figure a way to get the new sheath through the back of the "K" section of the pan and exit it somewhere near where it is supposed to be (or used to be).
This is needed in order to make the angle of the cable between the pan and the engine approximately the same as it should be, otherwise it'll bind as it enters the tube going through the fan shroud to the throttle linkage (or at the very least, it'll quickly wear a groove into the front end of the tube).
Because of all that, I would probably try running it through the tunnel. I don't know how it's supposed to attach at the front, but the old cable tube exits the tunnel just behind the gas pedal and you maybe can use the same location, snake the new sheath through that hole and through the tunnel and find a way to either get it positioned to exit through the same hole behind the torsion bar "K" section.
If that looks impossible (and many things on these cars seem to be) then run it along the floor, out the back next to the brake line, gently angle it up to the throttle cable tube through the fan shroud (leave a nice, gentle bend in it to compensate for engine rock), through the tube and you're (almost) done.
Is this clear or did I obfuscate?
gn