Hmmmm, putting heads, cam, and exhaust on a 305 would do it, but a crate, 300hp 350 from GM costs like $1200 and you'll get a warranty. Since the car was an original V8 car, the warranty actually kicks ass, almost like a new car. I'd say drop a crate 350 in and call it a day.

For example, my 350 is a stock '72 out of a 1/2 ton pickup. It's got a cam, intake manifold, and headers, and produces over 300hp on stock heads with 8.5:1 compression. It's pretty easy to get 300hp out of a 350.