With the addition of some aluminum heads(and a little more compression) and a camshaft, you could easily make over 600 hp. This will be the best bang for the buck. You could get a good set of aluminum heads(complete) with quality parts for a little over 3K. In order to get all of the compression that you could run, you would have to surface the block, since the pistons are quite a bit down in the hole. Just by surfacing the heads and running a thinner head gasket, you could get the compression to around 9.4-9.5:1. This would be enough to get you over 600 hp. Hope this helps.
Eddie