Best to use a high tensile bolt. Especially if using the proper riv-nuts as they are stronger than the commonly available consumer types needing more heft to compress properly. Made to be fitted with a hydraulic or hydro-pnuematic gun but good luck fitting one of those in unless you have the uber expensive extension thingy for dealing with confined spaces.
Two rigid washers with a decent film of grease between them will make the job go easier.
If I ever get round to fitting my sidesteps I shall use a ball thrust race to make life even easier. Gotta find a use for a least one of the four I saved out of the brake mechanism of the Lancia Beta HPE I had to break about 40 years ago. Too obviously useful to bin. Er. Ahem.
My steps need welding and I've not figured how to either safely get the rubber treads off or source suitable replacements.