I'm not recommending one grade bolt over another, just sharing what little I know. The bottom line is the user must select a bolt with adequate properties for the given usage, you decide from there. I will say that I can't remember grade 5 bolts being used in aerospace - it's been awhile for me, but I recall the standard being approx equivalent to grade 8, with other alternatives available. You have to realize there are steel or steel alloy fasteners with strengths a number of levels above grade 8, some over twice as strong. Selection of these higher strength bolts required a thorough assessment of the intended usage, whereas grade 8 equivalents were virtually a default with a generally acceptable level of ductility. Mil spec bolts, however, have much more controlled construction and thorough inspection than automotive/race car grade 8 hardware, so the probability of having an inclusion as Dan mentioned would be far less, as well as the underhead radii, etc would probably be more desireable on the mil spec parts. I recently had to buy some 1/2" "off the shelf" titanium mil spec bolts that cost around $40 each! This doesn't mean grade 5 is no good for Cobras - as Dan said original cars apparently came standard with grade 5 for most suspension areas. I must add, though, that guys who raced these cars in the day had no end to suspension issues - here's an example:
http://www.clubcobra.com/photopost/s...cat=500&page=1
So, what the factory did doesn't neccessarily mean it's ideal.