B-Body 8 3/4 Rear End in an A-Body

Good afternoon guys. So I've been debating about putting a B-Body 8 3/4 into my 68 Dart in the future. The A-Body ones I see are generally either too far or too expensive. I currently have an open 7 1/4 with 2.91 that was running fine when my car was running, but I'd like something that will probably kick it up a notch. I was thinking about something around a 3.23 to 3.55 gear ratio with a 25-26" tall tire. I was running with a stock 273 and 904 trans but hope to do a 340 or 360 engine swap sometime within the next year or two. I do plan on making the car more of a handling car. I have the KH disc brakes and 10" drums in the back with stock steel wheels all the way around. I was hoping, after the rear swap to throw on some 17 x 7 or 8" wheels with 4.25" and 4.5" backspace respectively. They make them in both sbp and lpb so I don't really have a problem. I know I won't be able to get too wide of a tire on there, but I was hoping for around 225/45/17" tires on the front and 245/45/17's on the rear for now till I can get better wheels.

Ok now that I've talked about my plans I can ask my question. Since bolt pattern doesn't really matter, how wide of a B-Body rear end can I go? I know the 65-67 rear ends are about 2.375" wider and the 68-70's are 3" wider. Is the 68-70 too wide? I also found a local 8 3/4 rear end from a 64 B-Body for $150 but I'm guessing that may be a little too wide for my Dart (3.75" wider). Also is there anything else that would be needed to do the swap besides changing the driveshaft length and relocating the springs 1" inboard? Thanks for all your help and have a great day!

Edit: Also found an A-body 8 3/4 drum to drum on CL for $500 not too far away. Maybe I should snatch that?