[original_balzer]

Not to rain on your parade but $600 isnt going to get you very far. Beside the obvious throwing good money at a worn engine, Horsepower cost money.

That being said heres what I would do if I was in your shoes. Buy a used rebuildable engine and an engine stand. You can then take the rebuilder appart and determin what needs to be done from there.
Try to find a long block with 010 cast into it 9 out of 10 times its a high nickle content 4 bolt main. This plan shouldn't be more than half your budget. A rebuild kit with some upgrade goodies should be less than $300 as well. Bottom line if your truck runs decent now live with it and take some time to build an engine you will be far happier with in the end. Take a little out of a few pay checks even machine work can be had cheap if you shop around. I personaly have build several 300-350hp 350's fopr under a grand and 400hp can be had for around $1000-$1200. Your local ad paper craigslist even evilbay can be your friend.