Well here's the deal guys. I just finished rebuilding a 9-inch rear for my '73 Fastback. I got it out of a '73 MachI so I didn't see any problem with her fitting right in. What I forgot to check is when I measured from the edge of the third member (where it bolts to the housing) to the end of the yoke the 9-inch is about 1 1/4 inches longer than the 8-inch member that's in her now. So I figure to switch from the 8-inch to the 9-inch rear I'll need to shorten my driveshaft 1 1/4 inch. How critical is that measurement, and what if I'm off an 1/8 or a 1/4 inch on the shortening? Originally I measured an inch and a quarter and was going to shorten the shaft an inch and a half to be safe (she sits almost fully into the trans tail housing now). Then someone told me it has to be cut exactly the right amount, no more no less. I've never had to shorten a driveshaft yet, how close must I be? Anybody with experience with this I would appreciate any feedback. Thanks.