Hi Marlin:
I for one would not use a 10 1/2 inch clutch behind a l-78 but everyone has their own way of doing things.Those doing it with no problems please chime in.I guess I just like my feet too much and would worry maybe not justified with a 10.5 taking on all of that HP, but that is me.
Here is the differences in the openings for the starter.
621
403
I agree with james on the starter snouts but remember the 403 using a very small end cast iron snout with the cross bolt pattern which is not common.Might be with you z-28 boys but I had a hell of a time finding one when I used this combo on a 292 chevy.
Hope the pictures help.