You’ve certainly done your homework on the truck. The Marmon Heritage book is a great reference. Your next task might be to find the M-H maintenance and parts manual for the truck, or more likely a copy of it. This is where Chuck Mantiglia might be of help, in addition to offering his overall superior knowledge of the breed. Oldest info I have is the 1939 parts list for the F-5/F-6 M-H models (no relation to later Ford F-5s or F-6s). That era might share some parts with your truck, but I believe I’m seeing that yours has the closed drive line where the later trucks had open drive lines so there are big differences there. Below is a link to Chuck’s Trucks if you’ve not already contacted Chuck. Stu
https://www.chuckstrucksllc.com/