My car is a '75. I ditched the HEAVY RBBB junk a long time ago. Finally found a good deal on some fiberglass early-style bumpers. (Thank you DAN ROOT!!! Now YOU have the naked car for a while! )
Ok, so getting to the mounting...
I didn't have measurement references for the location of the holes, no dogbones, there are studs on the bumpers... I have an idea about where the holes should be drilled, but the front passenger side sheet metal looks different than the driver's side. Probably had been tweaked and a new piece welded in or something.
Anyway, I thought of a trick that worked perfectly: I have a roll of very wide and thick masking tape/paper. Stuck a piece of that over the area where the brackets go. Hung the bumper on the lip and lined it up, then pushed the bumper firmly so the top stud would mark the paper. Then just drilled a hole through it. Measured the distance for the bottom hole.
Repeated for the other side. (Glad I re-measured the distance between the studs on the other side because it's a little different.)
Anyway, I stuck the bumper on and all the holes lined up perfectly first try! No oversized holes, no filing ovals and using big washers... I'm a happy camper.