Best BB bang for your buck?

One big block is not going to cost more to mod compared to the others since they use a lot of the same parts. $1,500 isn't going to go very far so I would look for something that already runs and bolt on a different intake/carb combo along with headers and possibly a different cam. It will cost the same to do this for a 383, 400, or 440.

The newer 400's and 440's are going to have lower compression. Keep that in mind. Personally, I would look for a '67-'71 383 2bbl out of a 4 door family car. That way you will have 9 or 10:1 compression and can do some simple bolt-ons to get 400+ horsepower.