I did the same thing on my 'Cuda. Bought a set of original brackets for front and rear, removed the extension fillers (that fill the gap between the later-style bumpers and the tail panel and grille) and got a set of perfect rechromed original '70 bumpers from a shop for about $150 each. Picked up new chrome rear bumperettes, bumperette fillers, and brackets from Year One. It looks fantastic and was very easy to do.

FYI, you will want to attach the bumperettes and brackets before putting the rear bumper on. I didn't have the '70 service manual, so I didn't realize this until I already had the bumper installed and adjusted. Real PITA to do after the fact. Also, you will have an open gap between the sides of the rear bumper and the quarter panel. '72-up had the plastic fillers to cover this gap, and the earlier cars had a small metal extension that filled this in. You can either leave it as-is, or make some sort of sheet metal extension.