What I am seeing wrong with the geometry...
The pushrod is too short. This causes the new arm to have a bad angle. Makes for a digressive ratio between the brake pedal throw and the master cylinder travel.
Long time ago I learned that the angle between the pedal pivot to pushrod pivot and the pushrod should never go over 90°. Even at full stroke. If the pedal angle sweep is 15°, the pushrod angle should start at 75° and end at 90°. Even a hair lower is still good.
With the angle you have (guessing, not a good camera angle), you are starting at ~100+° of angle. As you press the pedal the angle rises even more. This gives a better mechanical advantage, but that makes the brakes feel like crap. The effort doesn't climb as naturally expected, the stroke increases at a faster rate than the effectivness gains. You need a longer pushrod and correct the clocking of the new arm,
While I am looking at this, how are you locking the pedal to the shaft? Almost looks like a roll pin. Maybe a small diameter bolt? Not thrilled with that one. You can generate massive pedal forces in a panic stop. I've known people who have bent brake pedals in a panic situation. Even if you say you will never do that, what about someone else who may drive this at some time in the future? While at it, and brake feel, how springy is that rod? You are now making it a small torsion bar. Adding a spring to the brake system. What you are doing has been done before. The mid 70's Hondas were a right hand drive design and the left hand drive design for the American market kept the brakes in the original location. They ran a bar like you are doing to the passenger side where the master was at. But I recall it was a torsionally stiff, larger diameter tube (been many years so I may be off).
Going to a larger master isn't always the fix. I was running the GM conversion with a mustang hydroboost and the 15/16" master. Tossed ToddZ the keys and let him drive it. Got a compliment for good brake feel from him. I started with the "get a bigger bore master" and slowly progressed my way down to smaller and smaller masters, each time with better results.
Not the happiest with the angle the pushrod enters the booster either. While it works, I am not a fan of that much long term side loading inside the booster. I would normally like to see the booster lowered. But I see that puts the accumulator into the inner fender, and it already looks like the final finish is on that. Angling the master up a little in the front is another option. Or a little of both. I don't think you can completely straighten the angle of the pushrod entering the booster, but I think there is room for a minor improvment.
