Struggling at high rpm above 6000

I had the same issue in my 65 A-990 Hemi car back in the day. It took some trial and error with many things that have been mentioned in this thread. Nothing worked. 5500 rpm and it started misfiring. I spoke with Warren Barnett at Barnett Race Cars and he told me to upgrade the wiring to the MSD box and replace the battery with a fresh one. The MSD needs approximately 1 amp per 1000 rpm. The battery/wiring couldn't run the electronics on the car and supply the needed amperage for the MSD at higher rpm. I made that simple change and I have never looked back.