Don't use stainless, they are not very strong as a general rule. OK for looks, but when you are really counting on it use grade 5 or 8.
I would use hot rivets. I replaced about 2 dozen missing or broken 3/8" rivets on my 2 ton frame. It's pretty simple with an air hammer, a torch, some homemade tools, and a friend.
For the parts of my frame that I modified and wanted to bolt in so I could remove them I went to a big truck parts store and bought special frame bolts, they are grade 8, black, have washer heads and washer type self locking nuts to match. They come in 3/8", and 1/2" diameter as well as larger and perhaps smaller, that's just the sizes I used. Some new tractor trailer frames are assembled with them, no rivets.
