I just put the back bumper on my 71 Charger about a month ago and its very easy if you follow these steps. I was able to do it by my self with no help.
1. Those large washers on the OUTSIDE are foam weather resistant washers. (Like the material you can wrap stuff with for shipping.)
2. Attach the taillight housings to the bumpers first.
3. Attach the large support brackets to the bumper with the chrome headed bolts.
4. Attach the license plate light and assembly to bumper.
5. You will notice that the support bracket has 3 bolt holes and 1 stud. Put the foam washers on the support bracket. (I held them in place with a little contact cement.) You said you didn't have the original bolts, but they are 9/16" head with 1.25" metal washers (on inside of trunk) and the two nuts with the 1.25" metal washer.
6. Heave the bumper into place and while holding with one hand Push the wiring through the holes and thread on the washer and nut on both sides. Then thread in the six other bolts. Align the sucker and tighten them down. The holes are big to allow for alignment but once the bolts are tightened it ain't moving.
Like I said its very easy. If you have a helper to thread on those nuts, even more so.