Correct harmonic balancer?

I have a 1972 date 318 with a '69 340 forged crankshaft. My harmonic balancer that I got off the original 72 engine spun the outer ring. So I'm shopping for a new one. But I guess there is a difference in them depending on if the engine is internally or externally balanced. I haven't drove the car hardly at all since engine build. But it does have a mysteries bump and thump almost like its missing but not, I notched that up to the spun balancer. Any input is much appreciated! Thanks everyone!