I'm thinking of doing this with a 98+ F-body. I've read some articles that Steve (owner of the one in GM high tech) has written and it seems the difficult areas are motor mounts, and head/header setups.
As far as the heads, using 3.8L GN heads with GN headers make things very difficult around the heater box when you go to run a downpipe.
You can use FWD heads and make it easier, but finding TTA headers may prove more difficult than fabbing a downpipe.
I would imagion that removing the heater box would make things much easier, if your willing to give up heat/air.
HTH.