Not sure about the grade, but the head is certainly what makes them different. I usually find only one or two places where a normal bolt would fit. If you have a small block Ford, you will certainly have difficulty using normal heads on many of the bolts because there is just not room for them. I always tighten them, heat them up and cool them down and tighten again. Also, I recommend matching the head ports with the gaskets and headers. I do this by putting the gaskets on the heads and trimming them to fit the ports where any gasket material may be blocking a small portion of the port, then take them off and put them on the headers and scribe the gasket pattern on the header (if it blocks part of gasket hole). Then use a die grinder to trim out the mating edge of the header if needed so the flow is uninterrupted from the head through the gasket into the header tube. This can really improve performance where significant there is significant mismatching .