Not sure about anythign specifc as im not a 90 ownder, but in general there are a few opinions re nuts and bolts for exhausts.
Satndard mild steel tend to corrode very badly
Stainless get mixed reviews, I had two sets that were great (on racing car) and 1 set that although did not corrode, it locked up solid.
I have also seen copper ones sold as special exhaust fittings which I have not tried yet but theory is good so will do that if I need them for my SIII
Whatever you use a very good dollop of copper assembly compound when fittingis highly recomended.
Hope this help but would like to hear other opinions.