SBP to BBP?

I'm guessing the car needs a rear axle upgrade. But, if it has working front discs & a (hopefully 8 3/4") good rear axle, I'm not sure why you'd want to change it. The later brakes would be a bit better, & you'd have a lot more options with brakes & wheels, but the brakes should be adequate, & I'm finding a lot more wheels available for the SBP than I first thought.