I know what you mean, those lil 5/16 holes look awful puny. Funny thing is I have lifted the entire front of the car off the floor using a carb-plate bolted onto my Airgap.....
A rough calculation says a 5/16" bolt can carry 500# x 4 =2000# (10:1 safety factor) . A grade 5 bolt would be a little higher. Its not the bolt we worry about though.