470 runs out of steam at 5100 rpm

Double check all of the easy stuff before you start swapping parts. Are you 100% positive that the throttle blades are wide open? I've seen a ton of cars where the throttle linkage is not properly adjusted. A lot of people make this mistake. Have you checked your cranking compression? Leak down? Double checked your firing order and spark plugs and wires? There doesn't seem to be anything wrong with your engine design. That cam is big enough to make power, those headers are fine for your power level, those heads are bigger than you need but shouldn't be causing an issue. The compression is in the right range. So it has to be some sort of tuning issue. You are running out of fuel, or the ignition isn't working at higher speeds, or the lifters are collapsing, or you have some other little thing wrong. A poorly adjusted throttle cable can cost you 50 hp pretty easily for example.