Too bad you already bought pistons cause that is what that engine really needs is the right pistons to get that compression up. Once you raise up compression then bolt on 340/360 heads it will run good. But with those pistons in the hole and the big chamber heads compression will be low. Thinner head gaskets will help but compression will still be low. You can play around with some numbers here:
http://www.kb-silvolite.com/calc.php?action=comp