I used 3/8" bolts grade 8, fender washers, lock nuts. No wood blocks, and used 4 bolts per side on my 2021 Ram 6'4 bed. Yes it is tapered slightly, but not to the point of not being able to use 4 bolts per side. I did have to slightly grind off a piece of my fender washer towards the end of the...
I used grade 8 hardware, size will depend on your truck bed caps. Every truck has slightly different widths. I don't remember what size, nothing overly huge either like 1/2" bolts. A size below that, but I measured my side caps before deciding on size. No backing plate, because of plastic clips...
good luck selling a deleted diesel on Socal. I gave up, and wound up selling my deleted 2015 cummins to a dealer in Henderson NV. Got a smoking offer for it because deleted trucks out there are highly sought after. They didn't even ask for the DPF and all other emissions garbage.