rear bumper brackets for a 67 with leaf springs

[hpcgmc]

I ordered a set of rear bumper brackets to install a sport bumper on my 67 GMC shortbed with leaf springs. I can't figure how these install, without drilling a new hole in the frame for a mounting bolt - and even that doesn't seem exactly right.

Can anyone tell if they shipped me the wrong part, or am I just not figuring how to install it correctly? The invoice says "67-72 rear bumper bracket set SWB 4x4, 2x4 w/Rear Leafs".

Here are some pics. The top bracket in the pics is the new bracket. The bottom bracket is the one that came with the bumper, and I think fits a coil spring truck.