Aren't Mercruiser and BBF headbolts the same? So your studs should at least match the bolts. There is something to be learned from Volkswagen cylinder stud problems. Fat studs broke. Slender necked-down studs stretched and did not break although they do fatigue, are stressed into yield and thus are a single use fastener. Aluminum can pull up around a stud as aluminum is ductile. Using Helicoils on studs engages more aluminum, resulting in less stress on aluminum around the stud. Shear strength of die cast aluminum is commonly given as 25 to 26 thousand psi. Gray cast iron shear strength varies from 26 to 88.5 thousand lbs per square inch. As to the threaded length on the fastener, it is claimed that the load is such that the first few threads strip out and then failure proceeds down the fastener like falling dominos . I leave the conclusion up to you as I'm no expert.