383 Rev Limit question

When you get the heads off, you will have a better idea of whats going on. If it's around 11:1, the open chamber swap might be a good direction to go. If it's much higher, that swap might still leave you with too high compression for pump gas. The 915's are a much sought after head, due to it having the larger ports of the 906's, with the more efficient closed chamber design. The often encountered small exhaust valve is their biggest downfall. The open chambered designs have a better balance of intake and exhaust flow in stock form. It's probably only about a 1/2 point compression drop from one design to the other, and my guess at it making a bit more is based solely on the better airflow of the newer casting numbers. I'll try and figure out exactly how much compression you would lose and get back to you.