I agree with Pro70. Start with headers. Its the easiest and fastest upgrade.

It will free up some of the HP already in the engine. If your engine is tired(aka old and worn), it won't be producing as much HP as it should.

Does it smoke?

Do you hear any chattering when the engine is running?

If yes, you might as well go for taking the engine apart. Get a book on the 350. Learn how to set it up stock then after you learn that you will find things that you may be able to do to get more performance out of it.