I X years ago I bought a neat chrome step bumper only to see that it is 2"+ narrower then the painted step bumper.
With a rear mount gas tank, it's a real challenge finding the proper mounting brackets.

 

I've switched them back and forth for years. it's just a matter of making spacers and putting them on whichever side of the bracket you need to close the gap.

I have an NOS wide frame bumper on my narrow frame highboy now using the original brackets with bar stock spacers. you just need to go the other direction.

 

Is it the bumper too short, or is it the mounts too close together?

 

https://www.ford-trucks.com/forums/1...ep-bumper.html
https://www.ford-trucks.com/forums/1...ep-bumper.html
https://www.ford-trucks.com/forums/1...-brackets.html
https://www.ford-trucks.com/forums/1...5-f-150-a.html
https://www.ford-trucks.com/forums/1...row-frame.html
https://www.ford-trucks.com/forums/1...ep-bumper.html

 

The chrome step bumper is made fo a truck with a narrower frame. The rear gas tank is in the way of the brackets,

 

It would be that.

My first thought was cutting them out and rewelding them ... but being CHROME it'll screw the chrome up.

Might be able to cut the brackets out all together and bolt new fabricated "L" shaped ones into the chrome bumper using regular 7/16" or 1/2" smooth headed chromed bumper bolts (like on your front bumper). Then bolt those to the wider frame?

Just a thought. It would be like mounting a regular fancy non-step rear bumper. Could even start with regular bumper brackets and modify the faces to mate inside a step bumper.

Likewise, two pieces of 5/16" or 3/8" thick steel angle at least 2"x2" on sides cut to fit inside bolted to the bumper from inside would give you a start, space them so that long flats would extend to outside your rear frame ... even reuse what you have?

Just thinking it out here ...

• Forget the cross bar, it was temporary to prevent someone (like me) walking into a bumper bracket. The two brackets were what came on my truck with a regular rear bumper. I repurposed them, they are 37-1/2" apart inside to inside. You can see where I cross bolted them to my DIY step bumper. My hitch is not part of any bumper.

• This is inside my DIY step bumper. My welded in bracket plates are 37-1/2" apart outside to outside, they slide snuggly between the red brackets above. Instead of welding them into the bumper, I'm suggesting you could make bolt in ones. Once correct distances are set, then mark and drill holes crossways .... or weld to the long brackets.

Just a thought.