I think the register hole in the crank & the TC nub were enlarged in ~68 or so & you can use the later crank/early trans (like you are doing) but not the other way around & the gap may or not be OK as is but I think people have done it & there was an article in a van magazine where in addition they cut some slits in the flexplate so it would flex & not wear out the pump bushing (iirc, not sure, I have it somewhere). Doug Dutra of the old /6 club of America used to machine a spacer to take up that space & later wildcat auto was selling em (not sure if it is needed & or the /6 one is the same as the V8 one) but it must be desireable otherwise it would not have been fabbed by several individuals. I have one if you need dimentions.