I could be wrong, but it looks very much like the one that reciently was for sale at Deer Park Wash. If it is it was on a 1970's vintage Chevrolet 1/2 ton with a long box. They were for sale as a pair. I'd guess this Grandby is about the same vintage. Pods8 advice above is pretty good, just have them measure it.
Good Luck with your search....