[emanon]

You need a good base to start from and that would be either the Dart or World products 302 Block. I have a Dart 408 and it is a strong engine but I really like the World products "new" 6 bolts per cyl. block. You can use your existing heads but when you get serious you can go with the new World products 6 bolt heads.
World has some nice crate engines with a warranty but if your budget won't allow that at least start out with the bare block from them.

E