I don't know that the 78 GMC box will work for you or not.
Here are some pictures of one still in the original truck:
[IMG]http://thumb9.webshots.net/t/60/660/5/0/83/2260500830080251109WUTkZG_th.jpg[/IMG] [IMG]http://thumb9.webshots.net/t/68/68/5/2/77/2908502770080251109spabmp_th.jpg[/IMG] [IMG]http://thumb9.webshots.net/t/69/69/8/64/36/2364864360080251109gLFDqi_th.jpg[/IMG]

I don't have any pictures of it in the 61 ford f350 I put it in, thought I did, but don't after all. I only used it because it was handy, (left over from a truck I scrapped) fit reasonably well, and was about the right size/strength for the job.
If you think one will work then go look in the junk yards, they are plentiful, as all the steering gear boxes from at least 78 into the 90's look about the same (later ones are metric).
I would not buy a new one until you had a used one in hand to be sure it will fit your application.

In short, go shopping at the junk yard for a medium duty truck power steering gear box....

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
---All pictures---
"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-