The largest wheel Mopar put in the wheel wells of the old ones was 14 x 5.5. I know that a 205/70-14 will fit without problems. I've seen old A-bodys with SBP Rallye wheels on them so I know the backspacing on them is acceptable.
Using the 205/70-14 size as a target for height and width, you could for sure get a 205/60-15 under the wheelwell and possibly a 215/60-15.
The newer A-body tends to top out a a 235/60-15 in front and 245/60-15 in rear. This is on a 7 inch rim with stock backspacing.:snakeman: