if it"s a 72a like we had, I seem to recall it had an over-running clutch on the gearbox input, which should alleviate shut down driveline stresses. Also the gearbox has a bunch of shims under it to align it perfectly to the blower shaft--if the blower has been adjusted (if it is adjustable) you may need to re align the gear box. There is a double roller-chain coupler for some flex, but I seem to recall the gearbox had to be aligned fairly square to the blower shaft anyway. No matter what tractor, we idled them way low and then engaged the PTO and brought the machine up to speed. The one time we didn"t, the gearbox became shrapnel, even with the slip clutch and shearbolt for protection. Make sure there aren"t any bearings out and the shafts spin true. the shearbolt flanges may need to come apart and be cleaned and inspected to make sure they move freely to seat squarely together.