I'm going this right now, with the benefit of my XB coupe as a donor car. Yes you can lop off the cable pedal pivot and the new hydraulic pivot is nothing more than a longer brake pedal shaft that sticks out the side of the pedal box. The hole should already be there for the master cyl to bolt up, just the pedal to go in and you're right.
At a pinch, the longer pedal shaft could be fabbed out of a long bolt (I think they're 5/8") and split pinned each end. The cable pedal could be adapted if you're not bad with a welder, just need to find the right bits of scrap lying around and get creative. I know I've done dodgier shit than that before.
