1965 Valiant brake woes

I'm not gonna go out and get measurements, mostly because I don't have any GMs or Ferds handy to get the precise measurements; but "cobble" definitely applies.
I can't tell you what the pedal ratio is on a Ford, GM, Kia or whatever is, but I can pretty much guarantee it isn't the same as a Mopar. The pedal ratio is set up to give you the correct travel on a given master cylinder- a Mopar master requires just under an inch, about 7/8 IIRC.
Now let's say the master in question requires (random number) 1 1/4" of travel to pump the necessary amount of fluid to properly actuate the size braking system it was designed for.
Attaching it to the Mopar pedal that is designed to supply ~<1" of travel will NEVER allow the brakes to fully apply, giving weak braking and a spongy feel. No matter where you set an adjustable pushrod, you will not get a full stroke of the master cylinder. Unless you redrill the brake pedal arm to give you the appropriate amount of travel, the master will not supply adequate fluid to fully energize the braking system (wheel cylinders, caliper pistons)- and that is assuming that the systems were similarly sized (displaced hydraulic volume) to begin with. Too large of a difference there brings on it's own issues.
The opposite is also true- if the master supplies too much volume at a given stroke, it will result in a system that is overly touchy and locks the brakes the minute you touch the pedal, giving you no modulation or feel. It's simple geometry, and even I slept through most of that class.
OP, your decision to go with the parts that were designed to work together and be done with it is spot on.
True, those cobble systems don't care what make they're attached to; they can work equally poorly on almost anything. It's a rare occasion I've seen them work well (except maybe on a Chebby). Waaay too many threads on here already about trying to get those systems to work right.