How much does it cost to rebuild a Slant 6

Where are you in Georgia? I am in Jones County, just north east of Macon. I will be glad to build it reasonably for you. I use Reeves RPM Performance for machine work. They are very good.

http://www.reevesrpmperformance.com/

The slant six is never going to be a hot rod engine without some kind of power adder such as a blower, turbo or nitrous. You will be doing good to get 200 HP out of one. However, that will be more snappy even in a 4 door a body than you might imagine. It would be fin to drive and still get reasonable mileage.