The LS1 swap just needs new...everything. Wiring/computer/accesory mounts, and then on top of that, you gotta create some engine mounts.
I'm not even sure it bolts up to the stock tranny (although I think the 4L60E recommended max is ~300 hp anyway).

Without alot of work, the 5.7L would be the most straight forward swap...and if you really wanna get exciting, you can always bore it out to a 383

But either way...probably should rebuild the 4L60, or go with a 4L80.

To see the LS1 ZR2...clicky