changing bolt pattern

I am not a fan of redrilling axles or hubs. The OEM Kelsey Hayes hub doesn't have enough meat to safely drill to the bbp. I am saying that off pictures I have seen. A much better solution would be to use the modification found in the first link below.

Using mustang rotors for 4.5" bc

The link below gives the details on how to use OEM bbp pattern parts for your disc brakes. Included is a list of donor cars that would have your needed parts.

A Body Disc Brake Swap 101

As far as redrilling your rear axles, don't bother because the 10x1.75 brake drums are no longer available and there are threads constantly showing up with people trying to locate them.

Depending on what rear end you have would make a difference of options.

In addition to the B-body rear end mentioned above here are other options below.

If you have a 7 1/4 then you can swap a whole bbp 7 1/4 from a 1973 through 1976 A-body rear end in. That would take no modification of spring plates or drive shaft.

If you have a 7 1/4 then you can swap a whole bbp 8 1/4 from a 1973 through 1976 A-body rear end in. That would take different spring plates and modification of drive shaft.

If you have an 8 3/4 then you can get new axles that will be bbp and the correct spacing for modern brakes. You can use lbp backing plates, drums and related parts from any 8 1/4 that has 5 lug wheels. There is also other backing plates that will work but I am not going in to that not knowing what rear end you have. To get axles, I would recommend Cass at Doctor Diff.

www.doctordiff.com