Driveshaft length tolerances +/-?

Boy, you guys tend to over think things.
A lot of people forget the fact that when these cars were built, there was sometimes a factory variance of up to 1/2'' in some cases for body squareness and that affects the rear end's position in the car. FROM THE FACTORY.
That's why the engineers built in a ''tolerance'' for drive shaft length. (As well as other components in the car)
So, unless you are Ronnie Sox or some other racer that has modified the car substantially from stock, then using the tried and true method of loading the rear suspension of the car to where it would be at rest, installing the yoke in the transmission and bottoming it out, measuring center to center of the U-Joints in the rear end and yoke, taking that number and subtracting 1'' will work just fine.
When the suspension compresses or you load 600 pounds of groceries in the trunk, the drive shaft yoke will slide into the tranny a bit, and when you jump your car like the Dukes of Hazzard and unload the suspension completely until it tops out, the drive shaft yoke will telescope out of the tranny a bit, but it won't fall out.
Pretty easy IMHO and 1/4'' either way won't be the end of the world in this case.
You're not building a Swiss watch here where the tolerances have to be within a billionth of an inch.........