I have a box from a 78 GMC 6500 that may work. I was considering using it on my 48 in place of my IH Scout box, but with my diesel and other goodies it would not fit.
It may fit your truck nicely? Can you post a picture of the original setup?
Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-