Ok so theres a guy selling a nice running 5.3 vortec out of his 2002 yukon xl. its got the ecu all the wiring and box. hes sellin it for 450. thoughts of buying and sticking into my 84 replacing the 5.0?

It's basically an LSx swap, so the first thought is to read the LS1 swap sticky to see what you're in for.

Being a truck, you'll need to change the intake, all the front accessories, and the oil pan/pick-up. That along will double your $450 (or close to it). Then you'll need an LSx transmission (or adapt it to your 3rd gen with a flexplate spacer) and the other basics like mounts.

If you get an LS1 intake (pretty cheap), find a good deal on the f-body accessories and oil pan, you should be able to do the whole thing for around $2k (including the $450 for the engine/harness/PCM).