Has any of the internal rotating motor parts, rods, piston, crankshaft been changed or is it still all stock?
I'm trying to remember when the 1970 Superbird cars where made, not all the 1970 440 HP, especially 375 HP motors, came with the "6 pack" heavy rods. Do you know for sure that it does have the 6 pack rods?
You already said it had the 6 pack balancer on it, correct?
I don't envy you or him, this is not a good dilemma to be in I would take it apart now and have it balanced with the same parts your going to run in the car
I ended up with a V code 1970 Charger R/T 4 speed car that had a scheduled production date of 10-26-1969, a early car. That car had the original six pack motor replace under warranty in April of 1970, it still had the external balanced flywheel and balancer on it when I got it, the Flywheel was drilled with a big bit like shown in the pictures John added on here .
I wish that I had measure the diameter and depth of those holes, but I didn't