[CadVetteStang]

The 10:1 compression is from 74- 75 blocks with a true 8.5 compression. The 76 has 8.25:1 compression so my guess is that you will have 9.5 or 9.75 compression. You may even be able to run 87 octane gas with it if you use retarded timing and cold plugs. I know that Chris from the Cad Company does NOT like the 425 head on 500 block combo because of the port size differences. I can’t remember for sure, but I think the valve size is the same or very close. You might want to make a few calls to the build shops that specialize in caddy motors like MTS or the Cad Company. It is possible that your engine would do better with 8.25:1 pistons and ground down heads to pick it up to 8.5:1 with true 500 heads than it would to be at 9.75:1 with 425 heads. And of course, with 8.5:1, you can run the cheap gas at part throttle in the summer with no octane knock. I thought about the 425 head swap wit my spare 76 engine, but what I was going to do was put early 500 heads on it. That should give 11 or 11.5:1 compression. But it was going to be a low to mid RPM race motor in a light car with a long duration cam. I would not chance 11.5:1 in a heavy daily driver- especially with a short duration cam (I think something would break).