Hello all. Need some help with my friends car and its a ford but you guys are the best so I came here first. Buddy inherited 65 mustang with a 289 from his dad. Wanted more power so we rebuilt it. Bore was good so went with forged flat tops for 9.8 comp. Factory ported heads with 1.84 and 1.5 valves installed. Comp cams 282s solid cam. 230 at .050 and .528 lift with howards duel springs 135 seat 300 open. RPM intake 750 proform dp. 3400 stall and 4.11s. HEI distributor. Got it running took it out for a test drive everything seemed fine. Put on some easy miles then hammered it. Ran great. Would scream to 7000 no problem. Probably put on a couple hundred miles. He stopped by one day and said it was missing. took me for a ride and sure enough at 6300 rpm acted like it hit a rev limiter. Every time at the same rpm in every gear. Not fuel starvation because you can ease it up to 4500 and nail it and still 6300 misses. Running 6.5 fuel pressure at that rpm. Tried new plugs, no help. Pertronix in stock dizzy with matching coil with full 12v to coil. Exactly the same miss. Checked valve lash, all good. Spring look good but didn't check pressures but they all seem stout. Compression check good. I am stumped. Please any suggestions are greatly appreciated.