Go for the 8.5" The 10" appeared on the later 356's in the 60's.
Also remember to carefully straighten the script such that the bottom line on which the letters sit is straight. It's a pet peeve of mine that I'll see that someone went to great trouble to mount a difficult part to work with, and then the things not straight!
Oh, and I first made a template of a thin (1/8" or thinner) piece of scrap plexiglass (the acrylic stuff you get at Home Depot, etc). Place the script on it, locate the pin positions and then drill holes through the plexiglass.
I then put the script into the holes, position both so they look good where I want them (you'll find that wherever you want to mount them it is curved, making hole placement a bit more difficult) and then tape the plexiglass down to the car (if you make it 2-3" larger than the script the taping is easier). Remove the script and use the template to carefully drill your holes.
This way, not only does the script line up with the template holes, the holes are slightly angled correctly to match the shape of the body and the script lines up with the body holes as well.
I glued mine into the holes, but others have used TC's pin-lengthening method quite successfully (but you still have to drill those holes!)
I've often thought while building the current car, that it would make a LOT of sense for someone to make positioning templates for everything on the car and then sell copies of them to others. Things like the steering column hole in the firewall, Windshield corner posts, rear light positions, door handles, side window pegs, top bow pivot point, gauges (several variations) and dash switches, that sort of stuff. If I were going into low volume production I would have a set of templates to really shorten up the build time as a LOT of time is spent screwing around measuring and checking pictures and stuff.
There is a set of ancient windshield post templates floating around, but they're more suggestions than actual templates - they get you in the ball park, but you could be anywhere in the ballpark).
Just thinking........