I went from a 4.3 to an LS6 in my 2000 silverado, it was fairly easy. Good thing about your swap is the LS1 will drop right in and you won't have to change any mounts. Since I didn't have any front drive accessories with mine except for the waterpump I just bought the CTS-V accessory drive complete for 800 bucks from Jegs. Came with all the brackets and bolts needed, PS pump, alternator, AC compressor, tensioners, idlers and belts. Well worth the money. Same kit from GM was $2000! I think if you use the LS1 intake you have to do a couple modifications if you want to run the truck accessory drive. There is a write up here for it.
http://www.ls1truck.com/forums/front...take-swap.html
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.