lgowin-
I wouldn't worry yet about the plenum brace length 'cause you're probably going to have to modify it for hood clearance on a GTV
I'm confused when you say you're using the stock plenum but also spica throttle. Do you mean you're using the stock Spica throttle linkage bellcrank at the firewall? If so, you're on the right path BUT, you will need the stock TPS and AFM if you intend to use the original FI setup. Also, you'll have to fabricate a suitable throttle cable. The TPS is the black device bolted to the bottom of the throttle body, if you have it. This switch is pretty commonly available through numerous suppliers, and the AFM is common to some other same-era Alfas so finding a good used or rebuilt one shouldn't be too difficult. I can provide part nuimbers if needed. Keep hammering, and feel free to ask questions.