Stumped - Starting Issue with 1940 D30 GRD233

Is it possible that the starter might be having mechanical interference? Are the mounting surfaces on the starter and the engine flat and clean? Was there a spacer between the starter and the engine that may have been left off? Is it possible that the drive gear pitch on the starter may not match the gear pitch on the flywheel? The company where I worked had a starter Dynometer for testing starter output. Of course that was back in the day when starters were rebuilt in house.
Are the bushings (aka bearings) in the starter motor in good condition? Is it possible that the brushes may not be in correct orientation? Do any of the electric connections get hot when trying to operate the starter?

Just for fun. I always say, check the ground connections. Most problems start there. So, my son was being given some brain damage by his GF. His sister said, "Have you checked the grounds?" He was not amused.

cornbinder and Mr. Nutshop, if you guys are ever in the central valley of california I'd be happy to get you each a beer. I appreciate all of your help!

I went through all of the wiring, took the starter out again, cleaned all of the electrical connection locations, etc. Everything seems like it should work. I decided to take the battery to a different auto parts store. They checked the battery and after about 5 seconds... "the battery needs to replaced due to a bad cell". I got the new battery home, checked the starter off of the truck with success, got the starter reinstalled, tested the starter in the truck with jumper cables with success, and then hooked everything back up how it was in the first place. He fired up instantly!!! I'm so happy that he's running again. This is a great time of year to drive him around town. Thank you both for your help!!

Now you know why I prefer air start on my trucks with an air system, so much simpler. That is esp true when you have to rely on what some else is telling you (like when the 1st place said your battery was good). Glad you got it fixed.