Best way to go with rust, there's always more than what you see.
Here's some more info on the bumper issue. Early production model year '74's had shock mounted front and rear bumpers. Somewhere around December of '73 there was a production issue, and the factory started using non-shock (standard) front bumpers and brackets again. The August -December model year '74 cars only had the single mounting location for the shock mounted bumpers. After they converted back to the standard bumpers, the frame rails had all 3 mounting locations (shock mount location, plus the two standard bracket locations).
My '74 Duster was a June build, so, it has all 3 locations
View attachment 1715071382
So, other than the December '73 to the end of the model year '74 cars, the rails only have one set of mounting locations. Either they have the two standard mounts up front, or they have the single mounting location for the shock mounts. This shows what the other ones look like, although the tags aren't quite right. On the shock mount bumper the tag should read something like August-December '73 (for model year '74) and 75+.
View attachment 1715071383
At any rate, it would be easier to get a frame rail that has the correct mounting locations for whatever bumper your car has now.