Driveshaft length tolerances +/-?

To answer your question: the driveshaft (from stock length) can vary from 1/4" long to about 3/8" short from stock length. If you have any old D/S that you can cut up (like I have done) you can install the yoke (with u-joint) and then install the u-joint on the rear end and get a real good idea measuring from the center of one u-joint to the center of the other. If you don't have a junk D/S then you'll have to depend on a chart (they are out there) to find you correct length. Or you can ask for an FABO member, who has the same body and engine/trans/rear end set-up as yours?