[Jac Mac]

Yep you will break them with that many cubes, We/I built a 9" IRS for the Ondrack TVR [ classic racer-Hampton Downs/SFOS in feb each year] with SBF 346 cu in & it breaks 31 spline stubs out of the diff, as long as you use U-joints in the half shafts this will continue to be a problem, although going to 35 spline might help. To put it simply when you pop the clutch & the car squats at the rear the u-joints will not have equal angles which introduces a momentary mechanical bind & increases the actual load applied to the axles. you might be able to reduce this factor by repositioning the CWP unit height to minimise this effect, but that may introduce other problems in doing so. Best way is to convert to CV type half shafts to eliminate the bind.