I have done a few of these conversions.. So, here's a quick list of what *may* need to be changed/modified/altered/replaced:
1. Cooler lines (the later 80's have cooler lines that are spaced farther apart. 97 & later will have this. The earlier ones will have the lines closer together just like the 4L60E, TH-350, TH-700R4. You want the LATER 80e's though (97 & on) as they have some internal GM updates to make them a bit stronger than the predecessors.
2. Cross-member location will change as the mount sits further back on the 80. Think cut, re-weld, or buy after-market or universal cross-member kit.
3. Output shaft on an 80E is LARGER and has a higher spline count so you will need an 80 or TH-400 yoke.
4. Driveshaft length will change as the 80E is longer than the 60.
5. The wiring harness will have to be modified, or purchase an after-market harness, or re-pin the existing harness.
6. The PCM will have to be re-flashed to work properly. We have seen several attempts at local "tuner shops" to modify the 60e program but IT DOES NOT WORK. You MUST reflash the PCM.
7. The floor pan, or transmission tunnel, may have to be "modified" for the larger trans to fit.
8. On some vehicles that have the exhaust cross over, or under, or around, the transmission, the exhaust will have to be cut out for clearance and be re-worked to fit around the trans.
9. On some 80E's there are 2 speed sensors. You only need to connect to the one closest to the rear of the transmission. Your electrical connector may, or may not, reach the speed sensor on the 80E as it is on the drivers side, and the 60E is on the passenger side (1996 & later, earlier 60E's had it on the drivers side as well).
There are a couple things I'm forgetting but I'm in a rush.. Sorry.