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I've looked at my '70 grills. They are egg-crate and being solid chrome plated cast aluminum they difficult to restore. New repro trim parts are about $300 (?) each, so $600 or so to replace with new is pretty painful.
From what I can tell, the 70 egg crate metal grills can be replaced with the plastic grills used on the 68's and 69's. You'll have to do what probably is no more than some minor bracket fabrication to give the 68/69 grills attach points for the attaching screws. Of course, if you buy new 68/69 turn signal lamp assemblies, and the plastic grills themselves, I think you can see close to $300 for converting to all new 68/69 hardware. ...At least the plastic grills will never corrode, so you won't have to buy them more than once.
I just never liked all those little square holes. I'd much prefer the cleaner look of the 68-69 grilles. Are you sure they're the same dimensions as the '71?