Don't polished SS button-head bolts achieve almost the same effect for a fraction of the price? Functionally they are the same.
https://www.boltdepot.com/Socket_but...8_5_16-18.aspx

 

I just put my new fiberglass fenders on and used regular old hex head bolts. Stainless bolts and washers and nuts but nothing fancy. mine is not a show truck and I had a terrible time getting the rusted carriage bolts out of their elongated square holes so I decided not to ever have to go through that again. It doesn't look as good as the smooth head or allen head but works just as well.

 

I have to disagree with your assertion that they are "functionally the same". The bolts in your link have a very small head and do not distribute the load across a wide area as do the OEM bolts or the custom bolts/washers that have been discussed here. These bolts could be used but you would have to add a stout washer so that the bolt head does not dimple the bed side. There are bolts called "elevator" bolts that have a broad head that would accomplish the task...these too are cheap.

 

I didn't have that problem Charlie. Now I did use flat washers on the inside on the fender side. The 16 ga box side is pretty stout. I also used nylok nuts on the fender side. The holes in the box side were not original as I had to relocate the fender. The bolts fit the hole. I would agree with you on the washer if it were a round bolt in a square hole.

 

Nylock nuts are a GREAT preventative measure for bed side dimpling. I do paint and body work and when I do a bed the last thing I want to see is the owner install the fenders and torque the bolts/nuts down where the bed side dimples. It makes it look like I have done a shoddy job getting the bed straight. I recommend to the owners that they use nylocks and only tighten until snug with no dimpling.