Have you thought about the CNC combustion chamber AFR 305cc or 325cc heads? They seem to make great power and are a rectangular port head. I dont know how much you want to spend, but they are around ~$2500 a set. Here is a dyno test with these heads.
Here is a dynoed motor with the AFR 305cc heads. It made right at 700 hp. The combo is on the second page.