Help with 1972 k20 bumper bracket delema

[nitrosport_5]

Ive got a 1972 Chevy k20 longbed fleetside and im curious what bumper brackets are correct for what i need. i purchased these brackets (pictured) from a member here and was told they are correct for my truck. i never had a stock bumper on it so i had no idea what brackets i needed.

once i got them the holes on the frame didnt line up. the furthest rear hole would catch the bracket but thats it. after lookng at the truckandcarshop catalogue, it showed that the style of brackets i purchased were for a 2wd truck and that there was a set with further spaced bolt holes meant for a 4wd so i went ahead and purchased an aftermarket set of those brsckets. my frame holes line up just perfectly but the bumper fits like crap! its just a straight bracket with holes so i cant see how the aftermarket could screw them up THAT bad. it looks like if i used the first brackets i have it would alow the bumper to follow the contour of the body better but indont have a second hole for them.

here is a link to the page with the brackets http://www.truckandcarshop.com/trucks60_72/t60_013.html

so what style of bracket is correct? if the set i attached a picture of is correct why would my frame be drilled for the other style? its the original frame with number matching my cab that is all originally a 3/4t chevrolet 4x4. thanks