Have a 2000 silverado with a 4.3, looking to swap for 5.3. I have the complete engine, wiring harness and computer for the 5.3.
The wiring harness plug is the same for both engines.
Does anyone know if the wiring assignments on the plug will be the same, or what type of changes will be required.

You will need the motor mounts. Flywheel, harness complete for the 5.3. And you will have to redo the exhaust as well. The wire assignments for which plug. If you got the complet harness then just remove the 4.3 harness and plug the 5.3 into the firewall. And use the 5.3 computer. Read this NBS 4.3 to 5.3! (http://www.fullsizechevy.com/forums/showthread.php?t=144116)