Thank for the input and pics everybody. It appears the '74 bumper bolts are spaced further apart than the '73's are. E-tec's (I believe his car is a '74?) outer bolts seem to be closer to the end of the bumper than mine or ashley_j's, and the inner two bolts of E-tec's look to be closer to the license plate.
So I'm going to order a '73 rear bumper and fill in the rear valence 'shackle inspection ports' like e-tec did.
By the way, I had ordered the guards from Dave @ Roseville. Dave said install them or exchange them or return them, whatever works for me. It's good to know when you order a part that doesn't work out like you thought it would, Dave still takes care of you.