Steven,
I am no expert on the Timken split axles, and won't pretend to be so. Do you have any manuals for the GMC (or Chevy for that matter)? I have a digital version of the TM 9-801 for the CCKW 352 and 353 trucks and it has some discussion about split type axles and the brakes look just like the ones on your truck. I think this link is where I got the manual:
https://archive.org/search.php?query=subject%3A%22TM+9-801%22
Searching through technical and parts manuals will greatly aid in figuring out what you have and what is original to your project trucks.
Paul
