I hear what folks are saying about the stronger the better but in this case I think Gr5 is just fine. This may be a bit eggheaded but I am an engineer. Please forgive my mashing up of numbers, kinda a hobby.
The bolts are not repeatedly loaded so you don't need to worry about fatigue. The difference between Gr8 and Gr5 is that Gr8 has a ultimate strength of 150,000 psi where Gr5 is 120,000 psi in 1" of less diameter. Say you use 1/2" bolts the Gr5 bolt is good for 23,500 lbs, where the Gr8 is good for 29,500 lbs (in tension). So if we take this further and put into real world terms, say you are a 250lb fella. If your entire body weight was taken in tension on the bolts in a crash you would be looking at the difference of the bolt breaking at 92g or 120g. 92g is decelerating from 206mph in 0.1 seconds where 120g is decelerating from 260mph in 0.1 seconds. Either way you are a quivering pool of mush so I don't think the bolt will be a concern.
PS, in case anyone is wondering how to visualize what the 1 trillion we spend on stimulus would look like, picture this. The White House has a plan area of 55,000sq.ft. If you stacked up 1 dollar bills in the WH footprint it would be a stack 729' tall. Sorry, like I said eggheaded.
