How to measure the drive shaft..... not "for the ideal driveshaft"

Miranthis said
The car in question came with and 8.75 from the factory, but had the rear end replaced long ago when the SG malfunctioned. It got a replacement "from the yard" as is a 7.25 rear. No one knows if the driveshaft was also swapped or if this is the original shaft and I know that the original driveshaft was 49.58 in. and was 2.75 dia

The drive shaft for a 7 1/4 is longer by about 1 1/2"

Look in the parts book for your car and the length and dia are listed for the car, the rear and the trans.

Some years of cars with 7 1/4 rears had a drive shaft with an isolator in the rear. Meaning the rear part of the shaft was small enough to fit inside the front part with rubber between the two