You can get the information you're looking for here: GM Vehicle Information Kits
Click on 46 Chevrolet Trucks and scroll down to Page 18 of the PDF file it will open.
1/2 ton bed was 78" long inside and 48-1/2 wide inside. Height was 16-1/4" overall and 14" to the bend near the top of the bed side, both measurements from the floor.
I can't see the dimension from the rear of the fenders to the rear of the bed directly, but you may be able to figure it out from the dimensions given in the figures.

Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.