This thread really confused me. Maybe it will make it worse if I try to explain it my way, Fred already cleared it up. But I guess it's the difference between the shaft+yoke being too long vs. too short that's being discussed? It can be either. But the overall length should be such that when the car is at ride height there is ~3/4" between the end of the slip yoke and the tailshaft housing, as F said. If there is less clearance, I would have the DS shortened and not cut down the yoke, though I suppose if it a 1/4" or less it would be ok? Excessive clearance would mean that there isn't enough engagement between the yoke and the tailshaft. This could be a bad thing in terms of the potential for breakage.

Rich Krause

 

The yoke portion of the driveshaft at the tranny end should have an approximate equal length to a stock driveshaft yoke if it is a stock tranny. My 1350 style yoke had to be shortened by an inch or so to make it fit correctly. If it wasn't shortened, the driveshaft itself would have to be shorter, and the yoke would be sticking out too far at the tranny, which isn't desirable. Just make sure the length of the yoke is pretty close to stock and there shouldn't be any problems.
The clearance between the yoke and the tranny seal should be in the 3/4" to 1" range. This clearance is generated by having the correct driveshaft length when the shaft is built.