I would think the Champion Plugs are fine.
I'm really thinking the slow advance may be a key reason or at least one of the reasons your engine fall's on it's face during acceleration.
"CAR STILL BACKFIRES WITH OFF IDLE ACCELERATION." - yep, that is exactly what happens with too little and too slow timing advance!
Take action on your statement here:
"I also have the complete MSD kit for changing distributor advance springs / stops and lockout."
Try lighter springs, weights and maybe different stops. Here is a good video - info near the end esp.
https://www.summitracing.com/parts/msd-8464