Engine builders/Gurus, I need your help.

No apparent header gasket leak, checked several times.
Vacuum gauge does not indicate a burnt valve.

Sorry. I forgot to tell you about the combo.
383 stock bottom end.
440 Source Stealth Heads / Currently using stock rockers
Comp Cam XE275HL 275/287 525/525
Edelbrock Performer RPM
Quick Fuel 750 VS