Mongoo$e- If the crank was, in fact, internally balanced, and you are using the B&M "weighted" flexplate (the one with the part moon cutout) then you have added an external balance flexplate to an internal balance engine.

I am somewhat appalled at the lack of paper trail your engine has. When I built my 408 the balancer gave me a detailed readout. Bobweight, breakdown of each piece, how much mallory metal and the final readout of the recip assembly, etc. That way, if I ever needed to replace one component due to failure, I wouldn't need to start from scratch.