David,
Welcome to the Bolt!
Fumblin has covered quite a bit there, and gave you some good information, based upon his experience. He's had a few of these Art-Deco's (more Chevies) in various stages of deterioration. He's done well, both, in getting them to run, and with the E-Bay thing.
I'm of a lil different opinion on the fenders. I've got "glass" fenders on the rear of my 46, and don't believe it would make but a minimal difference in the value of my truck if any at all. It may turn out that's all you'll find.
You don't say what engine your grandfather used, when he replaced the original, so it would be difficult to comment on that aspect of the truck. However, I think you really need to determine what you want to do with the truck (long-term) and make a decision based upon that.
If this is gonna be a weekend toy, I'd get it running and safe and enjoy it. If it's something you plan on spending extended time in and putting some miles on, I'd consider upgrading the drive train to a newer pressurized engine, and perhaps a different tranny and rear end set-up.
At any rate, I would try and locate the best parts I could afford, old or repop, and use them. Since it is a 1/2 ton, the truck would be more valuable to you and a future owner with a standard box, if you don't already have one.
Here's a web site that you might gain some information from. It's designed for Chevies vs. GMC, but alot of the information and parts would be the same. Unfortunately, it doesn't have any production data for the GMC's.
http://www.brads41-46chevys.com Good Luck with the project, and post us some pictures. We love pictures.
Oh yeah, as for your last question, and I'm reading between the lines here... Yes, they're worth some bucks!
