Here's a thought of an idea, (please bare with me on this), What if I did put the 305 heads on the 400, achieved the high compression, and run it on E85 fuel? I know that I would have to at least everything rubber in the fuel system, but I also know that E85 loves high compression and that the alcohol would help keep the cylinder cool. (just a thought to keep from having to buy new heads that I can't afford)