Hmm okay so larger valve stock X and J heads flow about the same as stock magnum heads. Only with larger valves. I have a set of J heads that look like your magnum heads exhaust seats gone and intakes on the way out.
How many miles do your magnum heads have? I think indy still repairs the magnum heads 2-300 per head.
Volumetric efficiency has alot to do with getting as much air through a small port as possible. So 200cfm through a small port will make more usable hp than a head that does 200cfm through a large port.
Guess maybe it depends on what kind of hp you want to make over 400 and different or ported heads. Point of diminishing returns on valve size on a 3.91 bore is around 1.97" 4.02 bore is around 2.05 if I remember correctly. After those sizes the valve starts getting shrouded.
