Struggling at high rpm above 6000

Ok guys, thanks for all the feedback. Let me share some more details of how everything started...

Engine: Procomp heads with PAC SPRINGS ( Double springs with damper )
Solid cam .600 lift
Eagle forged crank, rods and pistons. ( Balanced )
Fuel system: Magnaflow pump, Pro systems 850 CFM carb
Ignition: MSD 6 AL Digital with MSD Blaster HVC 2 Coil, and MSD wires.
Champion RC12YC plugs.
I am running C12 mixed with pump gas.

Everything was running fine.

1 month ago, we installed a nitrous kit and a higher carb spacer....... ( the nitrous hasn't been activated or used )...... Here is where the problems started.

This weekend we tried:

Change from Champions spark plugs to NGK-7. We thought it was just fouled plugs.
Increase Carb jetting:
Front: from 76 to 80
Rear: from 86 to 88.
This helped a little bit....and struggling when almost away, but still there...
We changed the gap from 0.045 to 0.030, and the car felt on his nose passed 6000 rpm, It was literally like the car switched off...then came back to life......
We changed the gap from 0.030 to 0.045 and the misfire was there again, but didn't felt on his nose...