You could mill the heads down to get the compression ratio up a bit more.
Plus, I would not be surprised if the original thin shim head gaskets were replaced at the rebuild with thicker aftermarket ones; that is a significant compression reducer. Some thinner head gaskets are in order if that is the case. Of, course, you'll have to pull the heads to see ......
Good move to add headers. That will help all through the RPM range. Putting in longer collectors will help the bottom end torque a bit.
