I just did a 4.3L to 5.7L swap in my 2000 Silverado. The swap itself is rather easy, everything drops in and bolts up perfect, no friggin' around with trying to make it fit, as long as you have the V8 mounts it falls in place. I got a PCM from Nelson Performance tuned for my engine. Got a V8 wiring harness out of a 2001 V8 truck. Only thing I had to do is change the injector pigtails for the LS6 injectors.
2000 Chevy Silverado RCSB LS6 5 speed, LS6 short block, 317 heads, LS6 intake, Lunati 220/224 .575/.575 112LSA cam, Edelbrock Headers, CTS-V accessory drive, Nelson Performance Custom Tune.