1960 Apache C10 Step-Side Rear bumper / brackets

[60short]

Hey All,

I have a question about rear bumpers / brackets... I purchased a bumper from National Parts Depot, advertised as fitting 60-66 step-side trucks, and I purchased some brackets from Advanced Auto (keystone # : GMK414180755S but they were advertised as fitting 60-66 trucks (and their compatibility checker says it will fit my truck, but I can't seem to make them work, nothing wants to line up.

Has anybody seen this combination work, or does anybody know what these brackets fit?

These are a good 9 inches or so off of the inside of the frame rails, if I bolt through the bumper, and I've tried every combination of these that I can think of.

Are these off of a later truck?

Thanks,
Keith!