Grade of Bolts for Exhaust System

My 1991 Civic's Exhaust Pipe B (containing the middle muffler or resonator)
ruptured last week. I've made good progress removing the muffler and the old
pipe up to the catalytic converter. (Thank you, PB Blaster penetrating oil.) I
have a new pipe on order (and may end up buying yet another muffler, this time
OEM through an online shop). Meanwhile, I'm replacing several bolts and nuts
holding two exhaust pipe flanges together. What Grade is recommended? An
Autozone clerk said my old ones (probably from a Midas job) were Grade 3 or 4
and recommended at least Grade 5 or even Grade 8.

What's the design difference between grades? The material? Tolerance?

What grade do you recommend for these two exhaust pipe flanges?

 

The strength isn't all that important in that application.
It's corrosion resistance that matters. Get stainless steel.

 

Go to your local hardware store and get Stainless Steel Nuts, Bolts &
Washers.
It may cost you about $5.00 or so in hardware, but you'll NEVER have that
problem again.