Rear Bumper Mounts, why different sizes on 1985??

[pizzacutter]

So I found a great replacement bumper for my 1985 CK 10 (Chrome Rear Step Bumper) When I remove my old one I realize that the verticall mounting brackets on my bumper are 34" apart and the replacement is 42 1/2" ?? Everything else matches fine. The seller says it came off a 1985 just like mine... I guess they had different brackets from the frame to the bumper on this body style?

I don't know whether to seek out a new bumper or new brackets?
Any help would be greatly appreciated

 

[firebane]

73-80 and 81-87 have different brackets.

Stepside vs Flareside area also different.

 

[bucket]

That's some wide frame spacing. Might have actually come from a van, or the seller didn't feel like giving you the brackets too. Aftermarket bumper, correct?