You can use grade 8 or better bolts if need to. Rivets I wish I knew where to get them and the dies.
The steel, Not sure but I would think any 70 KSI filler rod would be fine. And since you are around welding I'm sure you know its not the weld that is the problem but the surrounding metal, thats why I recomend the added strength, if its not going to be used to haul a load then it should be fine. I guess if it was me, and since I like strictly stock, I'd go with just the weld and keep an eye on it.
