Grade 8 is a bit more than needed but I use Grade 8 on pretty much everything. I am one to overdo everything and I act like I have OCD on things like this and need/want everything to be perfect. Plus like Don mentioned they have the gold coating and help from corrosion. On my 66 F100 I used big grade 8's (cannot remember the size right now) and used big washers underneath as to not have them pull through the sheet metal.