Mines a 76... well that area is, but here's what I did. I removed the rear shackle mounts, bumper and skirt. Then moved the holes on the bumper mounting plate forward so that the first hole on plate is lined up with last hole on frame. On mine, that position worked, so I drilled out the plate to match the other 4 frame holes, in that position. Cut off the excess plate, where the no longer used hole are, then re-assembled it. On mine, the skirt was stapled on, so I had to tack up some holes and grind it out a little. And if none of the factory holes line up well for yours, you may have to plot all 6 holes, I recommend using a template.
Ps.
I don't have a factory fuel tank, so that might have made it easier.