Welcome to our site, Andys!! I hope you find the info you need to keep your truck project going to completion. All we ask in return is that down the road someday, you share what you've learned. Today's newguy (to the site) is tomorrow's sage/guru/expert/teacher/etc.
Just a minor point but it might help you in your search -- The Advance Design pickups only came with one bed size (according to model):
3104 (1/2-Ton) -- 78 inches (inside length)
3604 (3/4-Ton) -- 87 inches (inside)
3804 (1-ton) -- 108 inches (inside)
So to call a 3/4-ton a "long bed" may lead one to believe there are also "short" beds available (and vice versa). There are not. 3/4-ton beds are not "long" other than that they are longer than 1/2-ton beds. They could also be "short" compared to 1-tons. But I belabor the point. Thus, any measurements for a different size truck will be inaccurate.
Concur -- the very nice and helpful folks at Mar-K will be your best info source on this. Or somebody with a 3/4-ton ...
Again, thanks for coming to our site -- hope you stay awhile!
John
