What Wally Said. A stock '47-'53 tailgate should be just under 50 inches to fit between the bedsides.
The earlier beds 46 and earlier were 48" wide inside. I have a later tailgate that has been cut down to fit an earlier bed. It was a good quality modification so I didn't notice when I bought it. Won't fit in my '51.

Well, it would, but it would leak around the ends.

[on edit] The snow has melted around my pile of stuff

, so I measured my original. 48-3/4" wide at the top roll. It also has 1/2" flanges on the rear edge that don't go between the bedsides.