Mine had a 8 1/4 and it always seemed like the rear tires set in and we're on a narrower track than the fronts. I had a 8 3/4 with a 3.91 sure grip from a 66 charger that I relocated the spring pads on and it made the rear tires sit centered of the wheel wells. Hard to tell with the new wheels on but the track looked right while it had the stock rallyes.
Part of the problem is that the bulging quarter panels give the impression of the axle being too narrow. Also, if this is a front disc car, the disc brake setup has a 1 inch wider overall track as compared to the drum cars.
The 68-70 B body 8 3/4" axle is an excellent swap. It has the desired larger bolt pattern and fills out the wheelwells using even a 15x7 wheel.
