Of course the rears wont lock. They be too tiny
Nix the metallic fronts and get some organics. They have a higher CF. It will throw your balnce off even more to the front, but at least you might be able to stop. Do as others suggested and experiment with a smaller master for higher line pressure too, to get the ball stopping, so to speak.
You are correct on the SS. GM screwed that one up. They used the same combination block on those cars as the disc/drum models, but then again, they weren't running manual brakes. The fix for the balance on them is to remove the metering and proportioning valves from the valve body and run straight pressure.
The hydraulics are first and foremost to try to get the balance right, and you should start with factory specs, yes. Ideally your rotors would be the same diameter to make things easier to figure. I go by thumb rules rather than the intense math.
Here's my take:
Bias is thrown by things like center of gravity. It is the basis for which you choose your brake sizes. I make a distinction between balance and bias. It's probably not really right, but it helps to do the set-up. I think of it like a teeter totter with an offset fulcrum. The fulcrum (bias) is off to the front, so you have to find a way to get it balanced (make the front brakes bigger than the rear).
Your hydraulics are 71/29, balance being thrown further front by the different disc size. There is little chance the rears will lock with what you have, even if you jacked up the back end, had an aluminum axle, and ran my in-laws in the front seat. The rears are too weak in comparison. Doing all of that would change your bias, but not your balance (by my definition). Each brake would still do the same amount of work, but you would have adjusted the car to match the brakes (moved the fulcrum backwards).
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The OE hydraulioc balance with 4wheel disc was 65/35, 4wheel drum was 62/38. Estimating initial bias in this way doesn't work with disc/drum, unfortunately, but I believe it was listed as a 61/39 balance. Like I said, I dont do all the math.
The closer to 50/50 the balance is, the more likely you will need a prop valve, which is a good thing (though you dont actual;ly wany 50/50)
Running comparitively large back brakes with a prop valve is ideal for street driving, rather than setting the balance to not need one, because the prop valve allows full pressure to the rear during light-normal braking.
I would look for front calipers with a piston diameter of about 2.5-2.6" for single piston/ 2 or 4 piston calipers would need piston diameters of about 1.65-1.75".
Manual disc brakes depend heavily on pedal ratio. Verify that yours is over 6:1. GM changed the pedal ratio on many cars in the 70s to about 5.5:1. If you have the lower pedal ratio, it is nearly impossible to get 4wl manual discs to work.
Changing to smaller front calipers will definitely require a smaller master cylinder. For your best manual brake stopping, you would use a race setup with 2 single port masters and a balance bar. That way you can use even smaller masters for the extra line pressure without sacrificing pedal height, and you can manually adjust the balance in addition to having a prop valve.
If you choose to stick to a stock style master, you may have to add a little pedal travel with a smaller master to be able to lock the brakes.
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You could just do as others have suggested and experiment with a smaller master with your current brakes. It might do the trick for you to be satisfied. Your balance is off though. The hydraulics and the rotors both throw it to the front. Anything you would do to make the balance better with just pad materials will make it even harder to stop.
Howe makes a replacement lightweight 2.6" piston front caliper. It's better matched to your rears than what you have (2.9"), making your hydraulics a 65/35 match. Master size would be 15/16-1" bore. Dual bar masters would be 3/4-7/8".
Another alternative is you could run a dual racing master setup with what you have, with 2 different size masters, and be set. 7/8" for front brakes, 3/4" for rear. This would effectively change the hydraulic balance by sending two different line pressures... the higher pressure to the rear. Not really the ideal setup.
Or just go to power brakes, and forget about having good balance, kind of like they did with the Impala.
There are a lot of ways to get the car to stop in a satisfying manner. It is much more difficult to get it ideal.
Now I'm tired.
