If your truck's bone stock, start with exhaust. There's no sense in doing any engine work with the stock pipes. It's like trying to drink an Icee through a coffee straw.
Your Vortec heads shouldn't need any work to support 350hp, but if you want a big lift cam, put in some LS2 valvesprings, Comp Cams retainers, and umbrella valve seals and you can run up to .530" lift. Of course, if you're going to have the head off, you might as well do a clean-up port job. If you want more than 400 hp, you'll want aftermarket heads.
If there's no wear in your bores, stick with the stock rods and pistons. The stock compression ratio of 9.4:1 isn't a bad place to be. 87 Octane? No problem. If you really want a compression bump, use a thinner head gasket.
A cam swap is going to be where you make the power. People swear by the GM Hot Cam, but at $500 I'd rather call a cam company to get a stronger grind and spend the other $230 on LS2 valvesprings, Comp Cams retainers, and umbrella valve seals.
The big question budget-wise is "Who's doing the work?". If you're learning to do it yourself, invest in some beer and pizza for that friend you know who built his own motor five years ago and it's still running strong. If you're paying a shop, that $3k isn't going to last long.
