Not sure why, but I changed a crack flex plate on my 340/727, in my van twenty years ago, with another stock flex plate and it has never cracked again (pulled the motor out last year and it was fine). Parts just fail sometimes. Never had to use off - set dowl pins, but if I were to have a vibration, that would be something to look at, just after a motor or transmission swap. Input shaft bushings or rollers are cheap, change them if not sure of its history. I have seen some ruff input shafts also, clean them up if possible before installation into the crank.
