I've also swapped out the 7-1/4" rear end for an 8-1/4 with factory 3.23 gears and factory 340 leaf springs (believe it or not from a 340 Duster I found in a junkyard many years ago)
This is a long shot, but was this a 71 340 Duster roller, painted Primer black, but was originally yellow? With the middle of each rear wheel well lip missing like someone needed some small patch panels??