Susan,
I agree with the previous post.

If the donor owner will allow you the entire bumper and not just the cover it's much less work and hassle to pull the whole piece. There are three nuts holding the bumper to the bumper shocks ... two shocks, six nuts total. They are accessible from the back side of the bumper. Remove those nuts and carefully pry the ends of the cover out of the brackets on the rear quarter panels. The whole bumper w/cover will come right off. You can clean the bumper with a degreaser pre-paint prep like PREPALL ... sand with fine grit (1200) wet dry sand paper clean with PREPALL one last time and refinish with Bumper Black flexible paint before installing it on your Brick. The bumper should look like new. Try to get the three narrow, trim pieces that run across the top edge of the bumper if yours are in bad shpae and the donor's are good. They simply lift straight up out of a series of u clips hooked to the body. You can remove them before removing the bumper. Pry them up with a flat blade screw driver.

Squirting a little B'laster on the nuts and bolts a day before the job will is not a bad idea. B'laster can be found at Walmart, Pep Boys and other autosupply stores. Great product for busting rust on all sorts of projects. Makes getting otherwise stubborn nuts and bolts free with a lot less sweat, strain and damage.

Good Luck.