Cudajim says he has power disks so I don't think the pedal will be too hard.
If you didn't also grab the proportioning valve from the donor car (it is separate from the junction block) then it is not surprising that the rear brakes lock up. You can get an adjustable proportioning valve from Summit Racing. You can insert it into the line leading to the rear axle without having to cut or flare brake lines if you buy a short pre-flared section from the parts store and bend it into a loop, so it goes Junction Block Rear Output > Loop > Proportioning Valve > Existing Rear Line. The proportioning valve will wind up roughly side by side with the junction block, where you can reach down over the fender and turn the knob, but it isn't readily visible. In my experience, you need to turn down the rear braking by about 70%.
