I am not sure what kind of load you are talking but just about any 1/2" bolts will work. I am assuming that the mounting plate will have 4 bolts.
grade 5 bolts have a proof tensil strength of 85000psi and grade 8's are 120,000psi.
For 1/2 inch bolts, thats 16,000 lbs per bolt for grade 5 and 24,000 lbs for grade 8. Heck, even grade 2's are good for 10,000 lbs tensil strength per bolt.
So to answer your question, about any bolt will work but since there isn't much cost difference, I'd probabally get the grade 8's. Unless you want to spend a small fortune and get something like an ARP with rolled threads. Like Rod bolts or main cap bolts for a engine. Those are usually around 200,000psi tensil strength.