The first swirl heads were the "X" and "J" heads. that's why they are so great. Larry Shepard wrote about how they accidentally stumbled on it while they were flow testing. The modern swirl heads were a direct evolved design from the early heads. If you can get or have one of the earlier MP engine manuals, that's where it's included. I agree that the magnum heads suck big hairy sweaty dirty balls. They don't flow worth crap without port work, the rocker bolts cannot take a lot of cam and valve spring pressure without snappin off. I know people run them, but I promise you, you can take an old X or J and run around the magnums all day long. Chrysler took a step back in time with the magnums as far as I'm concerned. Loosin the shaft mounted rockers was stupid. those are just strong as all hell even in stock form.