My question to any experts out there...how much will I devalue the truck if I do this?
Probably not too much, really. But once you do, it will no longer be a "restored" truck.
A top of the line restoration's value on a long-bed, probably with a small back window, would be between $15,000 to $20,000. ( I am talking about TOP of the line that would cost 2x what the end results' worth would be.)
As it's your Grandfather's old truck, there might be emotional motivations to restore it to the way he had it. Understandable....
You could do a decent job of restoring it, depending on it's overall condition, for around $10,000 to $15,000 and have a very good driver truck that would make Gramps proud.
If you start hacking it up to make it something other than what it started life as, that's OK too. Just understand that weather you Re-estore, or Rod-store, the average value will vary depending on the quality of how its executed and the budget available to do it.
Val