I have a '76 W100 and I also have a brand new bumper from Mopar sitting in my shop, but the two will not go together!

Here's the jist: Many years ago, my Dad bought a new front bumper for (then-his) 1976 W100 and never got around to putting it on. Well, I want to put it on this truck now (mine now), and the holes in the bumper are spaced differently than the original, so the brackets that I have do not work. Here is my question: When our dealer ordered this bumper, was it for a later model (81-93?) maybe? The original bumper has bolts spaced approx. 33" wide and 5.5" apart (top to bottom). The new bumper has the same 33", but top to bottom the bolts are only 4" apart. Would brackets off of a newer model, that fit the 4" bumper bolt pattern, still bolt to the frame of my '76? Has a three bolt triangular pattern on the frame for the bumper brackets.

Anyone done this before? Please advise. Thank you!

JS