Why go to the expense of upgrading to the front to 10" drums only to do a disk brake up grade latter? All you need from the 10" drum set up is the spindles.
Or as I said earlier and 7071DartGTReg just mentioned look for a 73+ A-body with disk brakes and get the upper control arm and spindle. You then have the choice of 10.75" or 11.75" factory rotors that are readily available at the corner parts store for $50 or so and rebuilt factory calipers for $30 or so. Note: the 11.75 brakes will require 15" wheels. This swap has the added benifit of giving you the larger 4.5" bolt pattern opening up a world of wheel choices at a much more reasonable cost than the sbp.
If you have an 8.75" rear it will already have 10" drums but they are narrow at 1.75" and have the small bolt circle. If you are going to build a rear end from pieces I would recomend getting axles with the B/E body offset then you can use the more readily available 2.5" wide 10" drums or 11" drums. If you don't have an axle yet then you may want to consider an 8.25. It is more than strong enough for what it sounds like you are building in your signature line, already has the 10" by 2.5" brakes and will be the larger 4.5" bolt patern.
