Finished the starter. Here's the modified high torque starter shaft (top) vs the stock GMC shaft (bottom). I shortened the shaft, cut a 2nd groove (didn't end up bushing the original groove, don't think it poses a problem). I also shortened the Bendix stop to retain the geometry (I cut the shoulder further back on the lathe) Note how the windings stick out further.
This posed a little challenge. The windings on the high torque starter stick forward more. Since the shoulder where the Bendix stops against has to be spaced the same as the standard starter, it results in the fork that actuates the Bendix rubbing against the windings with the modification.
Ended up making an 1/8" spacer ring, see picture. This spaces the fork away from the windings but throws off the geometry. I had to bend the top part of the fork forward somewhat to get the required travel.
Here's the Bendix with the spacer installed.
Here's the fork clearance with the spacer installed.
All in all more work than I figured but, the end result should be worth it, a high torque starter with the stock GMC body so to say. Bench tested the starter and followed the manual checks, then painted it gloss black.
Happy with the end result, hope it works as good as it looks!