So the issue with this is that you don't really know what you're running for caliper piston areas. The determining factor for how stiff the pedal will be is the ratio of the master cylinder bore to the caliper piston bores. 15/16" is a good rule of thumb for these cars with manual disks, but it still comes down to what that ratio is.
If the pedal is still too stiff for you, you can go with a smaller diameter master cylinder bore. Keep in mind that will lengthen the throw of the brake pedal, so you don't want to go too crazy with it or you'll hit the floor before you run out of pedal stroke.
The other thing is that just running a drum/drum tee your front/rear distribution is fixed, and it may not be ideal for your car. An adjustable proportioning valve would let you determine the front/rear split, which can effect that pedal effort too. That said, I have an adjustable valve and I'm running it wide open on my car, so it's basically just a tee. But my rear disk conversion is new and I wouldn't say I have it fully dialed in yet. And again, that front to rear bias depends on a lot of things, caliper piston area, rotor size and swept area, weight balance, etc, etc. So what works on one car isn't guaranteed to work on another, especially if you don't know what disk conversion you have in the back.
