CameronThere are some considerations:
1. The 4L80-E is definately longer. You will need to have your driveshaft shortend unless you had a tall lift.
2. The mounting pad sits further back and higher than the 4L60-E. Changes to your crossmemeber are in order.
3. The 4L80-E has an additional sensor in the front so the computer knows what the trans input speed is in addition to the output speed. The hardness also has a few pinout differences from the 4L60-E. A harness adapter and programming will both be required.
4. The transmission you obtain must be a 97 and up model. This is the newer case design and it has the correct bosses on the side for mounting the sector cables and electronics.
5. The new design case has a return line input located at the rear of the case instead of up front like the previous design. Your cooler lines won't work unless you buy an adapter for the rear input. This is Sonnax part #34918-01K
6. The 4L80-E is avaliable with either a fixed yoke or a slip yoke on the output shaft. (I'm assuming you are 2WD.) If you buy a trans with a fixed yoke you will have the choice of replacing your driveshaft with a splined one (which probably needs to be longer than what's practical) or you will need to pop the tailshaft housing off, remove the o-ring, and purchase a slipyoke.
7. 4L80-E yoke is 1350, not 3R. The best way to handle this is to chop the 3R end of your driveshaft off and refit it with a Spicer 1350 yokoe and joint. Use a SOLID joint - stronger than the geasable ones.
8. There are large mounting tabs on sides of the 4L80-E bell. These may interfere with your exhaust. May need to hack 'em off. That is okay.
9. You will need a dust cover for the lower bell housing/toqrue converted.
10. These are 3/4 and 1-ton transmissions - translation: any stock type converter you get with it is going to be very low stalling compared to what you're used to. First gear is 2.48 vs. the 60's 3.06, so you should budget in doing a converter swap as well.
11. The dipstick is not the same as a 4L60-E. Budget that in.
12. The 4L80-E is also available in a "HD" version. This version has a ball-bearing tailshaft housing and straight-cut gears internally. Not the best thing for quiet mannered operation, but it is a more high-endurance setup. Does a street truck of any sort *need" this really? no.
13. The flexplate spacer. I don't have details on this since I didn't have to look into it for my own 60-80 swap on an OBS truck.