LS4 swap shouldn't be that hard on a 97-03.... You could probably used off the shelf parts such as mounts and CV joints to make it work. Bolting it in is the easy part, hard part would be the wiring itself. I think to do an L67 swap in a Pre-Gen there is a 100 wires you need to splice from the firewall to the engine harness.

If you're gonna do it, get a complete drop out from a GXP, subframe, motor tranny, CV joints, complete harness, etc etc.