The 67 Dart, Valiant, and Barracuda all used the same brake parts.
Do you know if the brakes on the donor car worked properly?
Are you using 10x1.75 rear drums?
Did you get the following parts with the calipers and spindles
Disk drum master
Distribution block with the safety switch
Proportioning valve
Did you replace the flex lines?
Did you rebuild the calipers?
If you bleed the brakes does fluid flow freely from all ports?
With a helper pressing the brake pedal down can you turn any of the wheels.
Not firm pedal can be from improper bleeding, master still has air in it?
Also power brakes typically feel mushy vs manual brakes. But obviously they should stop well with less pressure.
I assume your talking about the top edge where the seal sits.
The stock bore will stop correctly. Unfortunately 1" disk drum masters are MIA.
So you will have to use a 15/16" or 1- 1/32"
Either will work fine.
Something you need to do is verify that all 4 pistons per caliper are free to move.
That the rear wheel cylinders are the correct diameter.
