I would never consider using any bolt like the originals. I but my fasteners at Pacific Fastener. They are the go-to place for stainless fasteners.
On the matter of rear fender bolts, I bought 5/16" fine thread socket-head button bolts in stainless. I used a fender washer on the box side and another fender washer behind the nut, inside the fender. It is important to apply a little Neverseeze to the threads before the nuts go on. I like to use Nyloc nuts.
https://en.wikipedia.org/wiki/Nyloc_nuthttp://www.neverseezproducts.com/bostik ... pure.shtml I always use stainless fasteners to replace the originals. While stainless is more costly, I would doubt using stainless adds more than $25 to a rebuild. When I say, "rebuild" I am referring to a total take apart for metal cleaning and paint. The cab is three sections held together with 56, 5/16" bolts. There is the floor section, the top and back and the cowl.
This link is useful to keep. It shows the sizes and thread count for fasteners.
https://mechanicalc.com/reference/fastener-size-tables I should mention. It is a good idea to not use stainless fasteners on steering and suspension. There are special stainless fasteners for those purposes, but they are very expensive and unnecessary.