We are in the process of replacing a 6.2 / 3L80 with a 6.5TD / 4L80, and there are many modifications that are required. This post is solely about driveshaft lengths. Measurements are from center-of-cup to center-of-cup.
Rear Drive Line: 25 1/4 in.
Front Intermediate Drive Line: 32 7/8 in.