I was hoping to be able to find a driveshaft that would work without having a shop build one or cut one down because it seemed like a simpler process and could likely save money.
I swapped my rear out from a 7 1/4 to an 8 3/4 in my 74 Dart Sport. Using the original DS and taking the measurements that the shop I chose asked me to take I ended up with 48.75 inches CL to CL on the caps.
The bill was a C note, they cut, balanced and checked it for run out, fit great and have had no issues.