broken input shafts (plural!)

[DrJaysBlown5.0Mod]

I have gone through 2 input shafts in the last 2 months on my daily street driven 2000 GT. The problem started immediately after I had the motor rebuilt, which now puts 480 ft lbs of torque and 560 hp to the rears. Still, this should not be happening. The first input shaft was stock, in the original T45? tranny and original bellhousing. The second input shaft was in a brand new T56 with new bellhousing. Both tranny set ups included a Fidanza flywheel, Spec Stage 3 clutch and drag radials. Each time the input shaft broke we notices the pilot bearing was worn out--after only 2-3 weeks of city driving! What are some ideas for what could be wrong and how to test for each particuluar problem? Thanks for the help!