In going back and searching
GMPartswiki again, the only non-automatic transmission starter listed for the 1972 GMC 5000-5500 series with the 350 V8 is shown as GM# 1108362. A look at GM#1108776 comes back as being a fairly generic starter used in many small block car applications, but pictures show those all to have different starter drive nose cones.
Further search under GM# 1108362 (Delco Remy) just shows as an obsolete number with many replacement numbers which are all listed as obsolete too.

I think if you take John L.'s suggestion a step further, and take both the old and new starters to your rebuilder, you could see if he can do the starter drive and nose swap to get a functional starter.
Come back and let us know how things turn out.

Good Luck!!!