Drilling rear drums

Those just look like redrilled stock axles, rather than aftermarket axles with the big pattern, intended for the big brakes. So basically you are stuck with the small brakes, but drilling the drums is all you need to do.
The offset is the difference between the wheel mount flange and the flange that the retainer bolts to. It's different between the big brakes and a bodies. The backing plates are different too.