8 3/4 for 73 dart swinger

For a Swinger I personally wouldn't use a 68-70 B body rear, although I would consider a 65-67 B-body 8 3/4. The 68-70 is wider than a 65-67, and because the Dart body style has narrower wheel tubs the 68-70 will be a tougher fit. With 17 or 18" rims it could still work but getting enough backspace on 15" rims would be tough, and pulling the wheels on and off would require dropping the suspension to full extension because of the low wheel openings. I run a 68-70 B body 8 3/4 rear in my Duster, but Duster's have more room in the wheel wells and a taller wheel opening than the Dart's do. Regardless, if you use a B body 8 3/4 you need to move the perches. B-body perches are 44" center to center, A-body perches are 43" center to center. Wallowing out the holes in the perches and forcing the springs laterally to match the wider perches is a lousy way to do things and will have consequences (less tire clearance for one). Yes, you can do it, but there's a lot of shadetree crap that you can do that's still the wrong way to do things. And if someone says they've done it and not had "issues", they just haven't noticed the issues, which is not the same as not having issues. There's no way to avoid the decrease in tire clearance, that's just a fact. Side loading the springs will have a negative effect on handling, which may go unnoticed, and the spring bushings will wear out faster, which probably also would go unnoticed given that most people don't log a lot of miles on their A-bodies anymore. Doesn't mean it's not happening though.

E-body 8 3/4's would not be a good choice to narrow, the axles have a tapered step down so they can't be re-splined. The housing can be shortened no problem, but you'd still have to buy new axles. Most C-body or truck 8 3/4 axles can be re-splined, so if you want to save some money you can have those housings shortened and the axles re-splined, which is a little cheaper than buying new axles.

Depending on your build, you may not need an 8 3/4. If you want to keep in Mopar, you can use a 73+ A body 8.25 rear axle, or even an F-body 8.25" rear axle. For a moderately build street car you'd do just fine with one of those. The F-body 8.25 axle will be a little wider, it's about the same as a 65-67 B body 8 3/4 rear for width, but it would still work if you moved the perches.

Here's a list showing the 8 3/4, and some 8.25" rear axle specs including width, perch locations, etc.
An accurate 8 3/4" rear axle width list