What is the most likely reason my engine is missing at high RPM's?

The car is a '68 Firebird with the original, but fairly recently rebuilt Pontiac 5.7L 350, 4 speed trans, upgraded to 4 BBL carb/intake, and HEI distributor from a '75 (previous owner installed, don't know much about it). This has only started in the last month, and is not really all that bad, however, it's enough for me to take notice. Typically, it only seems to start happening after the engine has been running a while and is completely warm...the most noticeable and the worst being specifically at pretty high RPM's when I'm getting on it and taking off wide open throttle. I started a simple process of elimination by checking the plug wires, which are brand new... they were all properly connected at both ends. I checked the plugs, also brand new and correctly gapped, for any build up... they were all clean. I reset the timing around 6 - 8 weeks ago as well. I'm about out of ideas, and am wondering if maybe a higher performance dist and coil such as PerTronix or MSD would fix this problem(?). I had planned on eventually making the switch regardless. What else could possibly be the cause?