I have never personally heard of a small block crank with a big block snout. Your machinist is correct a blower can put a lot of added stress on the crank snout. Since SBC snouts are pretty small, you will want to run a short water pump (this keep the pulleys, etc.. from hanging out too far and creating more stress). What I chose to do is run a crank hub and no balancer/damper this allowed me to get rid of a lot of weight on the snout. Call a few blower shops and get some input before you buy any parts. I personally don't think you need a big block snout, if you have a good strong (forged) crank and keep the stress to a minimum.