Actually it does stop, but not the way it should. I expect this car to have a short travel pedal with a firm feel and "grabby" brakes. An easy touch should bring it to a stop, and a little extra pressure should make stuff fly off the seat onto the floor.
I get long travel to a firm pedal, but at speed (50 mph for example) it takes a heavy foot and something like 100 feet to bring this car to a halt. Maybe 50 feet, but much more than it should. I have:
Replaced the rear wheel cylinders, rebuilt the front wheel cylinders, replaced sections of brake line as needed to complete repairs, inspected/cleaned/adjusted all 4 wheels- pads and hardware.
I have replaced the master cylinder with an after market part and the booster with a (universal fitment) generic part from Summit. The MC is a Dorman product that looked exactly like my old Bendix MC. The booster is an 11" booster from Summit, a generic part but one that is supposed to fit. And in fact, it appears to be a very good fit. I had to fabricate a bracket for pedal throw.
I bled the brakes alone (all by myself) by putting a rubber hose on the end of the bleeder and the other end into a bottle. As the fluid fills the bottle the end of the hose is in the fluid so it can't suck air back into the wheel cylinder. I bled all 4 wheels until I was getting fresh brake fluid out of the wheel cylinder.
I've got about a 3:1 pedal ratio- it's pretty much the factory set up. I didn't drill holes or move the MC or the pedal.
I've got about 15" on the vacuum gauge at the end of the hose off the manifold.

Any suggestions why this doesn't stop like it should?

 

Mix-and-match parts come to mind. What is the bore on the M/C that came with that aftermarket booster? Factory pedal ratio is about 5:1 for power brakes and 6:1 for manual brakes. Are the brake shoes properly arced to the drums?

 

I can't speak to the bore specifically, but holding the 2 parts side by side, they appear to be identical. The booster is a 9" booster, not 11" as I mistakenly stated.

I checked the vacuum today- I'm getting 14" Hg at the hose. I took the M/C off the booster and re-checked the "plunger" length on the booster. The plunger is a screw so the length can be adjusted to match the size of the M/C. I set the plunger length such that, when I install the M/C and slide it in place by hand all the way until the plunger bottoms, there is still a small gap- less than 1/8", but enough that you can see there's a small gap. I closed it up by tightening down the mounting bolts. To me, this kind of pre-loads the brake system, and makes sure I don't have a gap between the end of the plunger and the bottom of the bore, a distance that would have to be closed by brake pedal before actually compressing the brake fluid.

As for the pads- I bought a set of brake pads when I replaced the wheel cylinders, but the current pads have nearly as much meat on them as the new ones, so I re-installed the original pads, and put the new ones on a shelf for next time. Wheel cylinders were leaking pretty badly- could the brake shoes have become saturated with brake fluid, making them "slippery"? Is that a thing? I notice the right front- the one that wasn't leaking- is the only wheel that locks up when I stand on the brakes. Could it be that the shoes don't grab as well because they were soaked with brake fluid from the leaky wheel cylinders?

Hmmm- I may have just answered my own question....

 

An 11" booster provides 50% more stopping power for the same pedal pressure than does a 9" booster. You need to MEASURE the bore diameter in the M/C. That has a HUGE effect on brake effectiveness. Contaminated brake linings should be replaced immediately and everything cleaned with lacquer thinner. And as I said above, when the drums are worn or turned, the brake shoes need to be arced to fit the new, larger radius for maximum contact and stopping force. You absolutely do not want to preload the master cylinder piston with the pushrod. The M/C has a compensating port that is uncovered when the piston is fully at the rear. This allows new fluid from the reservoir to enter the brake system as needed. Preload can prevent this port from properly being uncovered, causing additional braking problems. Kinda sounds like you have the perfect storm going here.

 

Thank you for your expertise, Joe. I have pics from when I did the job which show all the measurements I made at the time. The old booster was also a 9"- I simply misspoke earlier. I measured the bore diameter at the time, but I don't recall what it was. I'm quite sure they were the same, but I still have the old M/C so I'll check it again. As for the pre-load, I'll have to get my caliper/measuring device and check the distances precisely to be sure my "plunger" contacts the bottom of the bore, but doesn't compress the piston. That's an easy fix.

As for the brake shoes, I have a local shop that can turn the drums for me. I have the rear shoes and I can get front shoes, so I'll just have to take it all apart again, and take my time putting it back together. The brakes were never right on this car from the day I bought it. I really don't know what to expect from them, but I feel like I should be able to lock up all 4 wheels without putting my foot through the floor.

 

Follow JoeP's advice.
You really don't want to easily lock up the wheels. You want progressively more braking force as you press the brake pedal. ABS was developed for newer cars the increase braking without locking up the wheels in various conditions.

You may have someone to turn your brake drums, but can they "re-arc" the brake shoes to uniformly contact the brake drum ?

In my opinion,
#1. you don't want to turn the drums to maximum diameter.
#2. you want the brake drums (front left and right) OR (rear left and right) turned to the same diameter withing .005".