Hey guy theres alot of backposts regarding this swap throughtout the LS1 conversion forums. Anyways I have a 91 1500 it had the 4.3V6 with the auto tranny with the SHORTBED. I dropped a 02 LS1/T56 tranny combo in it. Its a really easy swap if you got the tools the time and money. For me the biggest pain were the clutch pedal and the wiring harness. Everyone will tell you the same get the motor, tranny, wiring and PCM all together.
Reuse the driveshaft, crossmember, wiring for the lights and accesories, I reused my radiator but Im going to upgrade soon just to be safe.

Buy a high pressure fuel pump (I used a vortech), fuel lines (hose shops will hook you up if you show them the fuel rail and filter for about 60 bucks), electric fan, motor adapter plates and thats about it. I reworked my own wiring harness and though it works its an awfull sight when you open the hood. Well good luck man anymore questions post.