I think you would have tons of room with a 9" wheel and a 5" backspace. The 73 has 9" rear wheels 4.5" backspace using 285/40 17" wheels (26" tall and 11" wide) and if I remeber right, there is at least 2" of clearance between the tires and springs. The same holds true on my 64 and 66 b bodies. The inner clearance isnt a problem, just the outer wheel well. I dont think you would need a sping relocation kit unless you were tubbed and had a narrowed axle.
Here is a picture...
It's pretty easy to measure your backspace and wheel width...
just plum a line down from the spring and the face of the drum where the wheel mounts. That should be your max backspace measurement. Then plum a line from the wheel well and measure from that point to the drum line. That measurement plus your 1st backspace measurement can be used to figure your max wheel width.
Of course you'll have to subtract about an inch off both measurements for inner and outer for tire clearance.
I do agree with hotrod98 though. My coronets driver wheel is over 1/2" closer to the spring than the passenger. This may be why we see a lot of this type of question and no one really has a solid answer. Different shackles, different tire heights, different inner wheel wells, different spring wear, misaligned quarters and patch panels. Too many variables.
I may be talking out my a ss though. I do have a few beers in me, but I'd say a 9" wheel with 5" backspace and a 11" tire will fit just fine,
I'd say do your measurements and go a little conservative on the rear tire width. If your smaller rear tires clear by a mile you can always burn them off and head down to wal mart for a new set of wider tires...........and then burn them off until you get it right
