I put a 409 in mine. The tops of the frame rails needed to be trimmed just a bit. I put a Turbo 400 behind it and used Sanderson headers. I did not put in power steering and did not change anything else on the truck. Eventually a 12 bolt posi should go in.
A big block should be fine. A small block will be fine as well. A lot of the dismantlers have the 5.3L LS motors from the new trucks for very cheap money. The redone wiring harness and computer actually costs more than the engine. In my neck of the woods, I can get a 100k mile 5.3 with reworked engine harness, with the 700R4 behind it for around 1400 bucks. With 300 HP of fuel injected goodness, you're not going to get a better bang for the buck than that. And 22 MPG to boot and it'll fire up on the first key turn. I haven't done one yet, but that will be the direction for the next one for sure. Good luck.
