Diagnose instead of throwing parts at it. It's cheaper.
Vacuum readings?
Propane torch (unlit and barely seeping) along intake-to-head parting line - look for RPM change.
Crossover allowing exhaust gasses into intake?
Carb circuits plugged? Carb over-fuelling?
Hate to ask - but I have seen this before - you are not numbering the cylinders like a Ford (1-4 on left bank, 5-8 on right bank) when you check firing order are you?
