I dont think its balance related as you look to have the stock rods and forged crank. Stock flat balancer? Get it shaking at idle and start pulling plug wires one at a time. See of anything changes. You could be losing a cylinder due to a wonky distributor bushing or something. Your O2 sensor will tell you if you lose a cylinder as it will go rich without combustion. Are you absolutely sure you got 5 and 7 plug wires correct? Bit me twice!
