You have an 02, 02-04 I6s should be the easiest to swap a V8 into. Easy to get a GM PCM in there, GM already did it for us! Run a 1MB LS1B PCM out of any 03-07 fullsize truck, find an 03-04 5.3L TrailBlazer pedal assembly and throttle body, full flash an extended TrailBlazer OS into the PCM, retune it for your setup and call it a day. The electronics ARENT that hard for 02-04 (maybe 05s), its above that when you get into the CAN based stuff where it gets tricky. You *might* have to have a dealer reflash your BCM with the firmware of a LWB TrailBlazer, but I cant verify that.
The radiators are different between the I6 and V8s, Id get a V8 one if you want to do the swap. Easiest way to do this would be to find an 03-04 5.3L LWB TrailBlazer/Voy that has been rear-ended and steal all the needed parts from it. Doesnt mean you have to use the 5.3 though, a nice cammed LS1/2/3 would sit nicely in there too.
Ive put this out there before and will again, if anyone in the NW wants to attempt a V8 swap in a truck originally with an 02-04 (05 maybe) I6 I will donate a few hours of time and an EFILive license so you can get it running (VIN change, VATS, engine displacement changes, etc).