I don't claim to be an expert, but here's my advice:
1. You'll need to adjust your transmission crossmember to the location of the new transmission. There may be holes already on the frame for this? Driveshaft my need to adjusted or swapped too?
2. You need to figure out the wiring difference between the auto and manual. Maybe you'll need a complete wiring harness that has that big connector that goes into the transmission on the 4L60e.
3. You need to reflash your ECU to recognize and operate the auto transmission. Or a matching ECU for the auto.
4. I suppose it goes without saying that the clutch pedal, etc. needs to be removed, unless you want to leave it as a theft deterrent?
Make sure you get the correct 4L60e for your year/ECU. Look into the sticky link that talks about differences in years.
And note there's a TSB for some wiring grounds that affects some 1996 models.
There's plenty of guys going manual, should be the opposite in your case, minus electronics.