It's not a drop-in swap. You will need motor mounts to adopt the LSx motor to the C3 chassis, oil pan mods may be needed, you'll need to come up with a transmission, since the C5 used a transaxle and I don't think the earlier transmissions bolt up to an LSx. You probably need headers for the exhaust system. Fuel system would need to be modified to feed the high pressure fuel injection system. Plus probably other details that would need to addressed. Stating with a F-body LSx motor might be easier to swap than one from a C5. Nothing that time & money wouldn't take care of but again, not a drop-in. But it would make for an awesome marriage.
I actually found a very interesting article on google about an ls swap and this guy was able to do it for under $2000. I'm assuming thats minus the motor lol. I also read that people cut a hole in their firewall for the computer but I saw a youtube video where this guy mounted it on the left fender wall for easy access.