It is a myth that anything is wrong with grade 8. They are stronger in all respects. Gr8 is not brittle, both Gr5 and Gr8 are ductile, medium carbon steels. I have bolted suspension components; and I used grade 5 that I had laying around. (Both are always available). If kits come with Gr8, I use them. If 4 tons shear strength per side is not impressive, use 8's. Waste of money, but not much money. Sounds like you will feel better using 8's, that's OK. For the bottom Rivets: Jack it way up, use stands, grind off heads, leave enough to see the center (.020), drill 1/4 hole X 1/4 deep into rivet shank, stick punch in that hole and punch up. I could never get an air chisel to knock off head so I grind. Try not to gouge up the frame. Wear enough protection so as not to be distracted while grinding under there if you choose to do it that way. Be sure you have the leaf spring installed before you tighten this bracket down.
Weird mystery for sure. Maybe even a factory repair by an apprentice riveter or an incomplete accident repair at some stage. Check the straightness of the frame in that area or other signs of collision.
