It actually isn't. There are plenty of guys who have swapped to a Th400 which is essentially a 4l80e. It's a very common swap on performancetrucks for guys who don't feel like doing all the wiring for a 4l80e swap. I've met 2 guys personally who had Th400s in their trucks and both were daily driven.No on the TH350. And you can use a 4L60E from the appropriate year, namely from the same generation truck with the LS engine (1999-2002) as it is computer controlled and has a different bellhousing.
It will become very expensive to try and retrofit anything else.