Help with bumper install

[**grumpy**]

I feel like a dummy, but I'm having a hell of a time getting my bumpers on.

I'm using my old front bumper on the rear of my suburban, because I was told by several dealers they are the same, front and rear, for the burbs.

Mine is a 53 BTW

I didn't want to take the brackets off, but I cannot get the holes to line up at all.
It seems to "spring" when not installed. I tried to pry it, and no go.

Is there a trick? or do I have to take the brackets off no matter what?

Thanks