Important plug gap question

[66StangGuy]

Problem: When secondaries open car stumbles. This only occures when secondaries open while in gear or when reving in park. when in park secondaries open fine and primaries fine. only when i open both at the same time the motor boggs. The last thing i did on the car before the problem is this. Installed new cap and rotor, New MSD 8.5mm cut to fit custom plug wires, new motorcraft resistor sparkplugs. stock heat range. Also running msd coil. used to have the box too but it took a crap on me. i also set the plug gap pretty high. i know my coil can make the gap no problem and it starts and runs great. only problem is when i give it a lot of fuel and air it seems to miss 2 cylinders then fire maybe 2 and then miss 3 or something like that in no certain order. if i leave the pedal there and see if it will recover it does not untill i back off the pedal enough to close the secondaries. only thing i can think of is that the compression (about 9.0:1) is preventing the spark or the spark is not sufficient.

does this make any sense? any other ideas to try and find whats wrong. iv already ruled out the carb. its a new holley 600cfm vac sec. no vaccume leaks or anything abnormal.. it all runs great untill the secondaries open. but if i open the secondaries by themselves under the hood the motor revs fine no problem. power valve is assumed to be good because of this. what am i missing here guys?