#1 Whichever disc brake kit you get should have whatever the manufacturer recommends.
#2 if it's a stock A-body 8 3/4 housing, and the spring perches are in their stock location, it will fit into your car. The perches should be 43" apart. As mentioned, if you're changing over from a 7 1/4, you'll need the bigger shock plate and U-bolts. You'll also need to shorten your driveshaft.