The only thing that I'd heard as a
possible difference, is that the painted ones were a lesser quality surface finish - perhaps with the later 73-74 ones with the black textured paint.
That said, a painted bumper will probably need to be just as smooth to get a good paint finish, & the factory F & R Valances & L & R Rocker panels were just as smoothly finished as were any other painted body panel, & those too were in the same textured black paint for all 70-76 MYs - so I personally doubt the difference.
However, with ANY bumper or other chrome plated part, you'll want to spend the money to get the surface finished & prepped well, then to do "Triple Chrome Plating" - which is actually Copper-Nickel-Chrome for the triple plate process - & to have the plater do as many copper dips & polishing that base layer as it takes to get the surface "baby butt smooth" before they go on to the nickel & chrome layers, in order for it to look great as OE. This is true whether it was originally chromed or painted.
So get a good reference(s) for chrome shops, such as Rob gave above.
Tom
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