The biggest difference is in the mounts, which tuck the bumpers in closer--you'll have to lose the flexible silver bumper fillers panels. The only difference in the June '71-'74 front bumper is the jack slots, otherwise they're identical all five years. The rear bumper is different, with the '73-'74 bumper having more mounting holes, and the bumper guard holes were moved higher. The previous two offerings, '70-May '71 and June '71-'72, have less holes. All 1971 models built in June/July of that year have jack slots unless they had elastomeric bumpers.
When I was a wee lad of 17 with no funds, I tucked in my bumpers with junkyard mounts (remember those?), redrilled the rear bumper for the earlier guards, and eliminated the front "buck teeth" by simply installing a pair of 1/2"-long chrome bumper bolts. Perfect from afar, but far from perfect.