Have you called Southbend? I know of at least one dealer who, back when BMD was making clutches, contacted them and had them make a reduced force version with the same clamp force. They had no problem doing so, but maintained the same clamp force. So, it can be done.
But to answer your question, total force on the clutch and diameter are what's important. If you had a clutch that was several feet in diameter, you wouldn't need much force. If it was 6" in diameter, you'd need a huge amount of force to have the same holding power.
The reduced pressure 145 clutch is what was used in the 100's. They are great to use in low horsepower conversions. I don't have the exact data in front of me, but I think it has less holding force.