What clearance do you have between the top edge of your running board and the cab bottom?
I originally had mine mounted on the support brackets with a spacer between the running board and bracket to raise them for better alignment to the rear fender and thus levelling them out. Fender bolt holes were all ready punched in the new box side panels and the fenders are mounted as far down as I can get them.
I have the 3 inch wider fenders with the notch for the running board to sit in and they would have been down in the notch by at least 1 inch if i went direct onto the brackets.
Only problem is that I think that I am to close to cab because every once in a while the cab will hit the running board edge when the cab flexes with the frame i believe.
This is my 53 and the focal point was the wheel/fender in this shot but you can see the spacing under cab to board. These are the mid fifty 3" wider fenders, measure an 1-1/4" from top of board to cab seam. Mine fit tightly in the notch.