I know the 4T65E is the weak link in the GTP drivetrain. Has anyone investigated dropping in a 4T80E trans from a caddy as an alternative? I am fairly sure they are bigger so space might be an issue. Electronic control would also be an issue. is the caddy bellhousing different than the 3.8?

You are correct the Northstar has a different bell housing and there is no room to put the better tranny in. The new GXPs with the 5.3L are still using the 4T65EHD as well.