This seems pretty easy honestly? Just tedious, and easy to screw up by cutting off too much
1- craft a jig/scribe from wheelwell, for use later on to cut bumper to well
2- remove bumper
3- shorten length of bumper mounting hardware (brackets, bolts, weld --- as you see best)
4- refit to confirm mounting hardware length is acceptable, hammer in/cut off bits that don't cooperate as required
5- mark up bumper to trim ends using your jig, only take a bit at a time
6- refit to confirm cutout
7- repeat 5&6 eleventy-jillion times to fine tune cut
#3 could use some refinement, and some basic measurements wouldn't hurt I suppose