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.
