The heads are definately limiting you. I would also agree., that a 215 head would be good, if your buying new heads. They can grow with you, and are not to big. I have even used 230 Pro-1's a bunch of times on engines that most would say were to big for. One was a hyd. roller 383, that the guy bought all the parts for out of summit, had a short block built, and bolted it all together. He had some issues with the valve train, so, he took the heads off, I ported them to enhance flow without making them much bigger, chamber work, valve job, back cut, bowl abd short turn work, and ex. port. Picked the intake flow up almost 30 cfm, without taking much more than 5 cc out of the runner. By default, the velocity was way up, more flow thru the same sized hole. It was a tire fryer. Another one was a 380 inch destroked 400, with 230's, fully ported, to about 245cc, pump gas, solid roller. made 620hp on engine, ran low 10's on motor, and 8.80's on the bottle. No shortage of low end
I have also worked those Perf. Rpm's. Made 560 on a 406, with flat tops and a solid cam. ran 10's on pump gas.
Frank