With normal off the shelf stuff, something like the Saint brakes from Shimano are probably the best thing you can get right now and fit the biggest rotors your bike can take but...

If you are going to try to reach those speeds, you REALLY need to get comfortable with your front brake, you can use it at 50+ mph without issue as long as you know what you are doing. Road cyclists do it all the time.

You also should look into upgrading the frame to something designed around a disc rear brake.