Need help with rear disc swap

[BigRed76]

I'm looking for some insight here with my brake situation. I'll start with the details...

Front brakes:
Stock 71 11" rotors
Stock 71 2.935" Calipers

Rear brakes:
Astro van 11" rotors
Stock 71 2.935" calipers (same as front)

Stock 76 corvette master cylinder 1.125" bore
Wilwood adjustable proportioning valve (fully open at this time)

When I initially did the rear disc swap I was using the 76-78 Eldorado calipers (2.497" bore), and for the life of me could not get a firm pedal. The braking power was excellent, but the pedal was always a little spongy, even with the e-brake applied slightly. I then decided to put on some regular calipers, so I went with the same calipers that were on the front. Now the pedal feel is even worse. The truck has enough braking power to stop, but in a hard stop the pedal goes to the floor. I have manually bled and vacuum bled the system with no improvement. Seeing how the pedal feel has gotten worse with bigger rear calipers, I'm suspecting this corvette master cylinder is too small. Everything I have read about master cylinder sizing with 4 wheel discs seems to be a dead end as far as the end result. Any opinions on my setup would be appreciated. Thanks