72 Dart rear help. What do I have?
So what is the perch to perch measurement?
Doesn't really matter, neither of them appear to be A-body rear ends based on the perch location being on the bottom of the tube and the presence of shock mounts.
The overall width will be the main point, the perches are easy enough to move and will have to be moved regardless. Since we're talking about a Dart the rear end width can't be too much more than stock A-body, much more than an inch or two and the backspace required gets to be an issue, as well as getting the wheels on and off the car. For a Duster/Demon/Dart Sport/Barracuda, with the quarters being wider by about an inch per side, you can get away with a wider rear axle. Especially if you go 17/18's, since most have more offset off the shelf.
Ford rear end. Common swap is from an Explorer, most were 3.73 gears and many were limited slip. Readily available in wrecking yards, if you grab two "short side" rear axles and shorten the long side of the housing so both tubes are the same length you get a disk brake rear end with good ratio gears and a limited slip for cheap. Can be used as is too, mostly for Dusters/Demons/Dart Sports/Barracudas because in stock form they're wider than an A-body rear.