CJK440,
Regarding your questions, the 70-72 bumper mounting bolts are located in the same location.
You do not need to use the filler stip. However, you will have to plug or fill in the retaining holes as a result of removing the filler from your '73. Don't forget you will need to procure a set of the '70-'72 bumperettes or fill in the rear valance gap.
On the front, the '71 bracket is not compatible with your '73 bumper, same as the back.... You have two options. Use the '71 bracket and find a 70-72 front bumper. Or two, redrill your existing bracket per the prior poster's instructions. Use can use your front bumper and merely remove the large bumperettes. Just place a bumper bolt in the holes where the bumperttes were secured. The '70-72 do not have this extra hole.
The only other things that you may want to consider is on the front corners of the 70-72 bumpers are rubber wedges which swedged into the corners on each side of the front bumper. Your '73 does not have them. Perhaps you can add them if if it matters?
