Biggest rear tire in a 68 Dart?

OK, I just took a bunch of measurements a few days ago. I have a 1968 Dart with an A-body 8 3/4 rear an 10" drums.

12" = Fenderlip to wheelwell at lowest point.
11 1/2" = same as above, but 4 inches higher in the well.
5 7/8" = from Drum surface to leaf spring
5 7/8" = from Drum surface to inner fenderwell
6 1/8" = from drum surface to fender lip

Note, the wheel is pretty much perfectly centered in the wheel well. A 15 x 8" wheel with 4.5 BS is pretty much perfect, just remember that an 8" wheel is actually about 9" wide with the lip, so you end up with about 1 1/2" on either side of the wheel. If you install a 275/60R15 or 275/50R15 tire on it, it will have a total section width of 11", which leaves only about 1/2" on either side of the tire (optimistically).

Wheels that would also work:
15 x 10 = 6.5 BS (very wide, almost no room for sidewall)
15 x 9.5 = 5.75 BS (very wide, almost no room for sidewall)
15 x 9 = 5.5 BS (probably too wide)
15 x 8.5 = 4.75 BS (will give 11" sidewall w/275R60)
15 x 8 = 4.5 BS
15 x 7 = 4.0 BS
15 x 6 = 3.5 BS

My conclusion, you cant go any larger inside the stock wheelwell than 275, moving the spring inboard does not get you any extra room without minitubbing it. The clearance to the spring is not much of a factor, since the axle does not move in realtion to the spring, clearance always stays the same. The wheelwell and fender are a different story. As you drive around, corner, and launch your car at the strip, you will probably rub on the inner wheelwells, you can roll or cut the fender lip back about 1/2" for more clearance on the outside. Another trick is to mount a 275/50R15 on a wheel smaller than recommended like a 15 x 7" wheel, this will suck the sidewalls in and give you another 1/4" of clearance on either side.

Note, these measurements probably will work fine with 16, 17, zand 18" wheels also, just double check the MFG's sidewall dimensions as you can do the the "suck-in" trick with ultra short sidewalls.