Lots of cam and head flow, but perhaps not enough carburetor to feed the RPMs they want. With a heavy car, you may have been better off focusing on midrange torque than top end hp (??). With that said, Don’t let yourself get disappointed the first time out. It takes trial and error to find what makes each combo ET. That’s a lot of weight you’re pushing; you’re really going to have to have the chassis work well to get 4220 to ET effectively. My car goes 3820, and I thought it was a load.