If I were to swap in a 99+ T56 into a 98, would there be any surprises, or will everything work out ok if I follow the guide posted by dragonZ28 (splicing wires, etc)?

I read you cant put a 99+ wiring harness with a 98 PCM ...fair enough.

Anybody have problems with a 99+ T56 swap into a 98 FBody??

 

the tranny itself will swap in no problem. I have a t56 from a 98 in my 01 car.

 

I put a 99 tranny in my 98 TA. It's a 97 Borg Wagner. Don't ask me why it's a 97...
Some say newer trannies are better, some other the contrary. Dunno.
The instructions posted by dragonZ28 are 100% correct. They worked for me.
Something I made, and it's not described in the tread, was to connect the reverse inhibitor solenoid to the stop lights circuit. Apparently it's usual to go that way.
I put a diode close to the solenoid to protect the stop lights relay.