Go to a good local fastener house, and get hi-strength threaded rod. I'm not sure why you feel the need for a grade 8 stud. I've used grade 5 all thread with no problems on gas engines. The combined fastener "joint" will only be as strong as the weakest component. If the stud goes into an aluminum prop hub, the threads will most likely pull out before the stud or threaded rod would break. My $.02
Z