1/2 ton AD truck frames are narrow at the front and much wider at the very back, not parallel and hard to say what width it is, more like a range of widths.
Illustrated here, note front width and rear width in chart.
http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm0202.htmThe 3/4 through 2 ton truck frames are 36" wide outside and parallel starting at the back of the cab.
The GMC 400 series and larger AD trucks are 34" outside the frame rails. From then on and still today most larger trucks are 34" outside the rails.
The frame that fits your truck best is the frame it came with, or the equivalent from similar truck in that model year range.
I'd suggest just driving the 52 for a good while and then figure out what you might want to change, if anything. They're pretty good trucks to start with. Give it a chance, might suit your needs just fine and if not you'll have a better understanding of what you do or don't like about it prior to making big changes.
