Valve / Piston Clearance in 383

I didn't read every word of the last 3 pages, so if I am repeating any info, I apologize. I have a 383 in my Barracuda with the stock style no valve relief pistons. Mine sit .020" in the .030" over bores. I have a lightly milled set of "516's", with 2.08/1.74" nail head valves. (the nail head style reduces combustion chamber volume). I can't recall what they CC'd to, but it was less than many report. With a composite gasket, my compression is at 9.97 to 1. The cam is just a Summit .465/.488" hydraulic with stock rockers. RPM intake, 750 DP carb with a Proform body. Race weight is about 3,300 pounds. I have a major issue with something, it won't make power past 5,000 rpms. Even so, it ran a 12.4 et the third pass at the track. Your combo should be making at least 450 HP.