I have a few nuggets for you.
First off, a manual front disc/rear drum A body can be a really impressive thing. I own 3 of them. They stop better than my '70 Charger with power 4 wheel discs. Pedal effort is great. The fronts skid before the rear like they are supposed to do. What helps is that the A body cars I have are a good 700 lbs less than my Charger.
Secondly, you have options.
You could use a 10" or even an 11" front drum/spindle/knuckle from a 66-70 B body car. You can even do it with your original upper control arms using the ball joint spacer that Dr Diff sells. These front drums would have the 4.5" bolt circle you want. You'd need everything between the control arms though...Drum, hub, spindle-knuckle and lower ball joint. Using these will require adjusting the tie rod sleeves because the B body lower ball joint steering arms in the lower ball joints are angled about 3/4" further toward the center of the car than the A body drum lower ball joints are. This same thing happens when you do a 1973-76 A body front disc swap into a drum brake car.