First off, I'd take a seamstress's tape and measure both fenders. Don't measure from the floor. Measure from point to points on the fenders themselves. I'd bet you find them to be identical. If they aren't then one of them is bent or the fender has been damaged and repaired incorrectly.
If the fenders are identical, you've either got frame problems or the cab mounts are rotted away or the fenders aren't mounted correctly, or something else like that. How do the fenders fit around the cowl and hood?
Start taking measurements of the frame on each side, up and down, side to side, and diagonals, etc.
Post some pictures of your truck here.
1947 to 1953 AD fenders are identical on 3100 trucks. The larger series truck fenders won't fit on your truck. Another possibiliy is that one of the fenders is an aftermarket replacement that's made wrong but I've never seen one that far off. Look at the roll where the sheet metal curves under at the front of the fender.
