Your main concern with that engine swap is the weight factor. Your suspension is not set up for that much pressure. Also, the entire drive-train will need to be replace, as well as the rear end if you want to get all your little ponies on the ground instead of just at the flywheel. If you've got the 4.3, it's much easier to beef up this lighter engine to get more performance.
A mild cam and rocker kit, tranny, carbon drivshaft and posi-locker will definately give you a kick in the pants from that 6 pack.
If on the other hand you've got the 4-banger...I'm sorry.
One other thing, all V-8 swaps for these trucks requires the removal of A.C. system.