If you put 70-72 bumper brackets on a 74 it just tucks the bumper closer to the body correct? eliminating the need for the rubber piece that runs along the back and the ones that go in the corners?

 

Personally, I don't think so. '73 was the first year for the front and '74 was the first year of the front and rear mandatory 5 mph bumpers on cars sold in America. I'd think that there is more to that bumper than just longer brackets but, I've never had one of those apart myself.