Mine came off the line in November of '73. The ones I was gonna get were off a '71, so do you think that would be too much of an issue? Could the holes be drilled to accommodate?
So, does it have the shock mounted bumper with the single mount?
If it does, and it probably does, if you use the '71 rails you'll either have to add the mount point for your shock brackets or convert to the earlier bumper.
It's true that the shock mount doesn't really do anything for you, but, the whole bumper is different. The non-shock mount bumpers have their bracket attachment locations in a different spot. So you have to get a set of standard (non shock) brackets, but you'd have to get the bumper that goes with them too. It may be easier to add the mounting point for the shock mount bumper.