I have a 67 SS with recently rebuilt BB 427. Compression is about 10.5-1, stock oval port heads with roller rockers, Lunati hydraulic cam .550 lift, part # 07202, and performer intake. Holley 850, Holley elec. red pump, Aftermarket HEI distributor wired directly to fuse block for 12 volts. Now the problem; It launches ok but at around 4000 to 4200 rpm it starts to stumble as if its running out of fuel, then progresses to misfire and back fire through carb. I swaped carbs with the Q jet I was originally running with same results. Then I swaped the distributor complete with coil. cap, and rotor with a stock GM, and still have the same problem. I know the cam was properly installed (I helped build the motor) and I readjusted the valves. Heads were gone through about a year ago and worked ok on the old motor. I'm stumped, ready to pull it and buy a crate motor.

Is it possible that you are experiencing valve float . Do you happen to remember what spring pressure is recommended for your cam and what you actually have. Checking closed spring pessure (at your installed height) and what it would be over the nose (at max valve lift) is very important. Depending on your cam choice, a spring pressure of 120 lbs closed and 330 lbs open is not uncommon for a flat tappet camshaft of .550" lift or so. This would be good to over 6000 rpm.
You did mention that you swapped complete ignitions so I guess spark scatter (the light show under the hood when the lights are off in the garage) might not be it. Sometimes plug wires hate being by firewalls and valve covers and the spark can be going to ground and you won't know until you look at night with the motor running and see the spark jumping. If you did use the same wires each time, try the dark room test and look for sparking. Sparking around the spark plug boot is common too and can be tricky to catch visually. Remember, something complicated can actually be something simple. I should know.

change your plugs,check your fuel pressure.what jets are you running?whats your total timing?

sounds like you know whats wrong .. acts like its running out of fuel ...whats fuel pressure ? is filter clean ? whats fuel line size ? more power needs more fuel your upgrades maybe more then your fuel system can handle