Before you cut that lower cross brace there might be another way to deal with it. The attached photos show a late model (48-53) F1 bumper and brackets that I used on the tonner along with a step/toeplate that was fabricated to fill in the gap between the bed and frame. The mounting holes on the (48-53) brackets exactly matched holes in the 40 frame-no drilling and were fabricated with a "kick down" so that the top of the bumper was almost on the same plain as the lower edge of the fender. Judging by your photo it looks like the top of the bumper would be right at or very close to the bottom of that cross brace. The plain iron bumper and brackets came from C&G Ford.
http://cgfordparts.com/