To some this question may seem to be weird. I wanted to shorten an aluminum driveshaft for my 65 coupe, which is running a t5 and a 58 9 inch rear end. I don't have the length, but my question to you is it not the same length as the stock 65 driveshaft.
It should be close enough to the length of a 65 manual drive shaft length, C4 shaft is a little shorter IIRC. But has the diff yoke been changed any time in history?
I just put a '96 T5 in my kids' '65, using the '65 bell, one of those aluminum spacers, and the '65 8". The '65 driveshaft was 51". Pushing the replacement slip yoke into the tailshaft and pulling it out 1", I found the distance from center to center of the yokes was 50".
Anyway, that's the way you measure it. With the car's weight on the axle, you push the slip yoke in all the way, then pull it out 3/4 to 1", and measure from center to center. Perfection is not require, but too long a driveshaft may spell disaster for the tranny.