This is a photo comparing front bumper brackets for a Challenger. The left is a '70 and the right is off of a '73. I believe the 70-72 and 73-74 brackets are interchangeable. Clearly the '70 bracket is shorter and the bumper mounts are different to accommodate for the larger bumperettes found on the '73-'74.

Regarding the weight, not sure how much difference there would be between the brackets. Perhaps a pound or two. I would think bare bumpers regardless of years would be nearly the same. The only other thing you may consider is to swiss-cheese the brackets to reduce some of the weight. You can eliminate the reinforcement plate located behind the bumper too.