High rpm miss

Thank u all. Here is what ive done today. Ran coil off of a relay has full 12v with car running. Lashed valves. Unplugged tach. Put old plug wires back on ran perfect with these just wanted new. Put old plugs back in. Changed carb back to 600 holley. Voltage good at high rpm. No rev limiter. Never had a cd box. Went back to hei distributor. Nothing helped every time any gear 6300 and will not rev a bit higher goes into total chaos at about 6350 and just falls on its face. Im thinking it has to be springs. Just weird, one day 7000 rpm next day 6300 max. This thing sings and sounds beautiful to 6300. I'll keep at it. Thank you for all the help I'll try some more suggestions.