Another thing is to make sure the bolts are straight - if the holes are drilled at an angle and you crank the head up against the mounting lug, you are placing tremendous bending forces on the junction between the head and the bolt shank and could induce a shear failure under the heavy vibration. I'd be more worried when using a tapped metal mount though. On a profile I would be tempted to fit some aluminum plates under the engine lugs. What do the lugs sit on, plywood? That's a big shaky engine to rest itself on two littl metal lugs on wood.

A single cheap grade #2 6-32 bolt has a maximum clamping strength of about 375 pounds. For grade 8 up around 800lb.

Stay away from SS. It has no value as a motor mounting fastener except for corrosion resistance, bling, or for a weak link. Jett uses SS bolts on their mufflers so they shear off in a crash.