[SCOTI]

Short of using the original cut off pieces/ends as a good template, I would mount the brackets to the bumper & snug them down enough to loosely 'install' the bumper on the rails. Use a jack or some sort of cribbing to set the bumper where you feel it looks right to/for you & then mark the holes for drilling.

This is how I'm doing my '64 (the body is actually sitting on a '68 lwb frame that was cut down to swb dimensions w/some additional mods). I even modified the bumper brackets some to work 'better' for my needs.

If you cut the frame down, this approach will be easy enough to get the bumper hung.