Suspension/Chassis Suggestions - 70 Dart Swinger
Correct, the 8 3/4 I have is suppose to be for an A body.
So for me, with where you're at with the build I'd honestly try to pick a set of wheels you like, and decide if you want to be able to rotate your wheels.
Some of that is just because I'm a bit of a wheel hound, but the other part is that its a big visual change and once you've put a particular rear axle in the car you're not gonna want to change it. And it will definitely dictate a lot of your wheel options.
The nice thing about already having an A-body 8 3/4 is that you already have the most expensive 8 3/4 housing, so, if you decide you want a 65-67 B body rear axle for its width you shouldn't have any issues selling your A-body housing and for more money than the B housing will cost you.
I would say the biggest thing to do would be pick your tire size and the tires you want. Then you can decide on the brakes, which will cement your track width (especially in the front). From there you can pick the wheels, and see if you can find something you like that will work with the A-body 8 3/4 or if not, what rear axle width will work with the wheels you want.
And that's not to say that you won't be able to find anything for the A-body rear, there are definitely options. But once you've got the rear axle chosen you're pretty much stuck with the offset/backspacing you need to work with it. Heck it even gets more confusing depending on the tire size you want, for 18x7's up to about 18x9.5 it's easy to find +30 ish offset wheels. Once you go past 18x10 suddenly most of the options are in the +12 to +25 ish range. So like for me, if I want to run an 18x11 in the back suddenly an A-body 8 3/4 looks like a better option than the 68-70 B. But under 18x10 the B rear is the way to go.