64 Dart Driveshaft Length

The driveshaft is a trunnion type and is original to the car. What was done is the 7-1/4 Rear End was changed to a 8-3/4 complete.
What is the best way to measure the driveshaft to see if it is the correct length now. I am not familiar with the trunnion driveshafts.
It is different than just center to center on the u -joint type that I know about.
Thanks Bob