You need a power assist if you are going to run 4 wheel discs. I takes more hydraulic pressure to activate the discs than you can apply with your foot. I would suggest going to a late model master cylinder and booster. It is important to use a large volume master cylinder with equally sized fluid pots for the front and rear to compensate for pad wear. You will need a proportioning valve to balance the system. You will probably also need larger diameter brake tubing to the discs on the rear as discs take a larger fluid push to operate properly.
Dave
I disagree that you need larger tubing to the rear. Chrysler didn't use larger tubing to front discs vs front drums. And P1V1=P2V2, so a larger tube doesn't improve performance if the MC is appropriate.
I can't speak for 4-wheel discs, but for front discs you absolutely do *not* need a power booster.
I converted my 68 Fury to 73-style discs, used the common aftermarket combination valve, and put the 'factory' MC on my manual-brake pedal.
MC is from mid-70s Dodge truck with manual discs. (I've never seen such a truck, but parts listings show it was available.)
I bedded the pads per some internet method, and I have decent 235-60-15 tires - it stops very well. Pedal feel isn't noticeably different than it was with manual drums, and I like the feel and connectivity between foot and the road. If I was converting another car, and had both MB and PB pedals to choose from - I likely would pick the MB.
Everything from here onward is my engineering thoughts arising from a 15-min consideration - so it's a rough draft - anyone reading must verify for themselves before installing to their car and 'publishing' it to a public roadway:
Rear brakes are not doing much braking vs the front, but you need to ensure enough fluid capacity. Once a suitable boresize is selected, the MC must be reviewed for 2 things - enough fluid volume to activate the brakes, and enough reserve volume to keep the piston submergered as the linings wear and the circuit becomes larger. You could compare this by calculating fluid volume of the wheel cylinders vs the calipers. This isn't perfect, as the friction lining wear must be taken into account - easy to calculate that into extra fluid volume on the caliper piston, a little complex on the drum brake as it might not be a 1:1 of lining wear vs wheel cyl piston extension? You probably could do it by simply extending a wheel cyl to its max and measuring full extension. Another option would be to get a MC and fill the chamber with brake fluid, then pour into a graduated cup.
To check your work:
Fill the rear circuit to capacity with the calipers/ and new pads NOT clamped to the rotor but in 'idle' position. Then press the brake to extend the pistons/pads and review the remaining fluid volume. The difference in volume is what the reservoir must handle in normal operation. Then you could remove the rear pads, install some appropriate shims to emulate worn pads, and see how much further the fluid level drops.
And if there is sufficient volume for operation, but gets borderline when pads wear to 75% or so - you could always top off the fluid level during a periodic maintenance check?