73 dart...B body 8.75...what size wheel/tire

Badsport is right, best way is to measure.

Also, it matters WHICH b-body rear you have, as they are NOT all the same. This isn't the holy grail, but here's a better idea about B rears

B BODY 8 3/4- Drum to Drum

'62-'63= 58 1/2" (And '64 Max Wedge)
'64 = 60 7/8" (Exc.Max Wedge)
'65-'67= 59 1/2"
'68-'70= 60 1/8"
'71-'74= 63"
'71-'73 wagon= 64 3/8"

A body rear is 57 1/8" drum to drum.

So, if you've got a '68-'70 rear, you're looking at 3" overall, or 1.5" on each side. Given that a 4.5" backspace is what you usually want for a 7" or 8" rim to get the tire the fender and without hitting the springs, I'd start around 6" of backspace.

But, that's just a ballpark. Dart's don't have a ton of space in the rear wheel wells, so if you're looking to fit the biggest wheel possible you'll have to measure.