Drive shaft length is different between the saloon (Alfetta, Milano or 75 depending on year and part of the world) and the GT / GTV6. Saloon is longer.
1) Import a GTV6 drive shaft since you essentially have a GTV6 engine and trans.
2) Shorten one of your existing drive shafts. May not be an easy task, but from what I have been watching here, that won't deter you. The shaft spins at engine RPM so the shortened shaft needs to be straight and balanced. You will need a shop that can dynamically balance the shaft assembly.
There are lots of fiddly pieces at the connection to the engine and transmission - bearings, bushings, springs. Plus there are several different sizes of rubber donuts.
I am sure that you can overcome this hurdle!
84 Spider - Long gone, but started the addiction; 85 GTV6 - Current project, Supercharged, Mega Squirt; 88 Verde - "Milo" It has a name, must be a keeper