I just put a 700r4 in my 65 Chevelle and I got a cut down drive shaft from a friend. The cut down shaft was prepared to go into a 72 SS with a 700r4. The shortened shaft is a full 8 inches shorter than the stock 65 drive shaft and, of course won't work with the 65/700r4 combo.

Is there a body length difference between the 65 and 72 Chevelle or was the shaft cut down incorrectly?

There is a difference, but I can't remember which way it works. I think the 64-67 is an inch longer? Try "Rambo", I think he has that answer.

I don't know about the driveshaft expressly, but all the early Chevelles had a 115" wheelbase. The 2-door Chevelles starting in 1968 had a 112" wheelbase (4-doors, wagons & El Caminos went to 116").