Did more digging on the forum, apparently there are 2 lengths of GMC V6 starters.
The longer starter allows for direct swap with a Chevy starter. All you need to do is swap the nose piece/nose cone.
The shorter starter (like mine) is the trouble child. The nose cone is shorter, as is the shaft and the gear.
https://6066gmcclub.com/showthread.php?t=17751
Low vs high torque starter explained:
https://6066gmcclub.com/showthread.php?t=47899
Rebuilt starter (maybe?) interferes with dipstick:
https://6066gmcclub.com/showthread.p...hlight=starter