Your Dad is probably right. If it's a '47, it's what's known as a second series '47, which is the very first of the Advanced Design trucks.
There should be a metal plate on the driver's side door post that has the VIN #. If it's there, post it here and we can tell you exactly what it is.
The motor is probably a 216 with babbit bearings, so it wouldn't be surprising to find the main bearings going out. Finding parts for it won't be hard at all. Just go to our links section and visit the vendors. Just about everything you might need is available.
The truck looks to be in great shape, so restoring it should be easier than many such trucks. I'm sure your grandfather (and father) will be thrilled.
Oh, and thank you for your service, Marine. This old vet salutes you.
