Dive shaft length on lowered truck

[dan42]

Got a question. On my '67, I'm gonna swap the motor and tranny out (probably a 383 stroker and and a th350) but I was wondering on how to measure for a new driveshaft. Do I lower it before I put the motor in, or after, and then measure for the drive shaft? I want to put a one piece in. Are there preset measurements? Not sure if I am gonna use a long shaft or a short shaft tranny. Preferably, I would like to put everything in at the same time, not have to put the motor and tranny in, then measure. Any advise would be appreciated.