Jerry - According to the 1940 Chevy Truck Shop manual, the 1940 1/2 ton's front "tread width" (measured across center of tire treads) is listed as 56-3/8". The front "tread width" measurement for the 1951 1/2 ton is listed in the
1951 Chevy Truck Data as 56-3/4". Difference of only 3/8" overall.
Not sure how much of a difference it may make, but the rear axle "tread width" for the 1940 is listed as 59" while the 1951 is listed as 61". Wheel offsets are the same for both year original wheels, although the '51's are 4-1/2" wide VS the '40's being 4" wide.
Below are screenshots/pics of the tread width for the 1940 and alignment data for both trucks for comparison in case it may help.
Edit: Joe beat me to the post button with the tread width data.
