1972 Front Disc Brake Conversion

if your front disks don't stop you it could be a number of things as stated above
i'll sling in a couple more...

if the pedal travel is short and the brakes feel a bit like wooden blocks your master cylinder bore is too great. you end up with little brake feel and the fat cylinder is not creating enough pressure at the piston in the calipers to slow you well enough with the chosen pad material.
a narrower bore will help provided that you have enough peddle travel

the pressure needed in the line to the front to actuate a disk brake caliper properly is higher than that needed to do similar for drums, this means that for the same foot effort what caused the drum to be full on, is causing the disks to be half on, and increasing foot effort means any problems with bias ratio between back and front is accentuated.

SSBC brake kits for B bodies used to suffer this. the master cylinder sent out was a mismatch in bore terms and therefore for the lever lengths in the peddle set up and many got on much better with a standard one

i.e your rears are hard on and trying to lock before you have the fronts applied in a manner that slows you down effectively.
Given that the front is heavy, got a motor in it, and can stand more stopping power before lock up the front should be doing a bigger proportion of the braking effort and you can't do that with the front pad not being applied to the rotor well enough.

in an Australian A body with 9 inch rear drums and small claw slider caliper rotor combination and a master cylinder designed for disk brakes. (similar to post 73 US disks set up) I had to fit a standard brake bias part, that chrysler eventually started fitting in Australia from 1975 onwards (it had been fitted only to police and race cars previously) in order to get my disk brakes at the front to really work properly, Chrysler fitted it after some criticism in the press. this car weighs tiny bit less than a 1968 dart but is shorter and had the smallest rear drums offered on any mopar and still the back locked up first.

i'm not suggesting you find a hard to get australian part that automatically costs way more due to its association with the Aussie R/T models and the less than popular 1975/76 VK range.

Summit for an adjustable one or a late model US part from rockauto would do if you don't already have one
things can also be made better with very narrow bore rear cylinders

its the thumb tack demonstration . pressure = force divided by area

push a thumb tack into the board easy with the fat end against your thumb.
turn it over and put the spike against your thumb and try and press the fat head into the wall and you quickly discover a problem.

Apply this idea to the various pistons in the system and you can't go far wrong.
brakes don't work well if the master cylinder is a thumb tack the wrong way round

Dave