Harmonic balancer question (with pic)

[1967C20]

OK, first let me say that I did do a search on this site for info on harmonic balancers and it did answer a few questions but I still need a little advice. My engine is the original ('67) 327 that came in my truck. In the picture below, what I assume to be the original balancer is on the left (rubber has deteriorated, don't want to reuse it). It measures 6-1/4" and has a thick profile. The AutoZone replacement balancer (middle) measures 6-1/8" and has a thin profile. A generic SBC balancer that I bought from Jegs (right) measures 6-3/4'" and has a thick profile, plus the timing mark is in a different position (I assume because the pointers changed at some time). Why the differences in diameter and thickness? Am I right to say that it doesn't matter which balancer I use as long as I have the correct pointer and it points to zero when cylinder #1 is at TDC?