I think the size varies between pickup with pickup bed, other trucks without pickup bed (mounted outside frame) and buses, larger and still outside frame.
You could do some more research in the literature and parts books to find out all the differences and where used.. However I think at this point safe to say you can fit a between the frame fuel tank from an earlier model AD pickup under your later AD pickup.
Keep the tank in the cab and with two tanks you get twice the range. I would like to do this on my 51, keep the original and add another tank.
Grigg
