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

Well, all I could get was a complete E body setup. BUT....there a local guy who cuts axles and housings, and does the re-splining.....all for $150. He's GOOD too. So now I can rock any width. I'm gonna take him my junk 7.25 with the 8.75.....and just have him match it up to the current width.
You still may want to have the Rims in hand that you will be using. You may end up an inch or two different than the 7 1/4 for a perfect fit. My Outlaw 2's have about a 5" backspace and are almost perfectly centred. If you end up with say a 3" backspace then you may want to go narrower when do it. I hope that makes sense/Helps you. Gerry