questions on narrowing a C body 8-3/4 axle for an A body

I'd have the same place narrow the housing and supply (or cut and respline) the axle shafts. If something doesn't fit, it keeps the finger pointing to a minimum. Take some existing measurements on axle flange to housing flange and let the shop know the size brakes you plan on using. I'd get the good axle shafts, seen bad results of adding horsepower to resplines.