Figure out what rims and tires you will be running. The offset of the tire can make all the difference in the world.

The stock 1946 fenders are about 11-12 inches wide. So the goal is to try to center the new wheels in the fenders.

On my 1940 GMC, I went with a 61 inch wide IMCA modified 9 inch.

I am using 16X7 artillery wheels with 4 inches of backspacing.

49 inches for the bed (bed probably closer to 48.5)
+ 8 inches for the wheels (4 inches for each wheel)
+ 3 inches for the tires overhanging the rims
+ 2 inches for clearance(1 inch on either side of bed.
= 61 inches

My measurements should center the wheels under the stock fenders for a 16 X7 wheel with 4 inches of backspacing. At the time I ordered the axle I was considering a 255/70r16 tire. I am now leaning towards a 235/85r16 tire. My truck is not together so I can't send pics of the finish.