Clutches are  like gear boxes, they are rated by torque capacity  not horsepower. How much snap or pounds of pull did you have on the handle? Also remember horsepower is a relative thing. Without doing the math if a clutch is handling 60 horse power at 700 RPM and the same clutch is handling 60 horsepower at  550 RPM the load on the clutch is greater on the 550 RPM clutch. If horse power is equal and RPM goes up the torque load goes down. The Belliville washer set up in the 880 PTO clutch is of a larger capacity than the one in the Super 88.