I have a '56 Chevy with a modified 292 inline 6 that has been running well for several years.  The car, in street trim with headers open, has run a best of 13.50 at 100 mph.  Last fall I installed a 9" rear end and all of a sudden I've eaten 3 clutches in 30 days.  Everyone I have spoken to says the rearend has nothing to do with the clutch problem - but it's the only thing that has changed.
The pressure plate is what is failing.  Centerforce tells me it is from "negative loading" which rips the straps/rivets that hold the face plate to the housing.  This, they say, can come from missed shifts - which has happened but it happened before the rearend swap w/o incident - or severe wheel hop which the car doesn't exhibit.  I don't know what to "fix" or where to go to ask the questions.
Anybody out there with an idea?  

sometimes "wheelhop" can also be classified as rearend flexion.  as the clutch is disengaged and the gas is applied, the rearend may be displaced, pivoting negatively. as the next shift comes into place, the springs positively replace the rearend and the clutch is then disengaged , etc and flexion. not necessarily causing the wheels to "hop" but causing the rearend to flex in such a manner as to displace the output power in such a way as the driveline feels the brute flexion causing the damage to the rivets in the housing on the centerforce.  after my 69 chevy went through about 3 clutches, i found a little extra money to replace the stock rear setup with a 4-link and this solved my flexion problem once and for all. all youre really trying to accomplish is getting the minimal "roll" off of the rearend.  

J - Thanks for the input.  Since the old rear end (original) didn't exhibit the problem using the same springs and traction bars could it have anything to do with the fact that we did change the pinion angle some when the 9" was installed.  We gave it about 2 degress negtive (down) angle.  The original rearend actually had a positive angle which I understand is not the way to go.
Again, thanks for taking the time to provide some direction.