when I did my last axle on my open deck trailer, I went with a dexter galvanized coated torsion axle, I couldn't find a good piece of something to stick between frame and axle mounting plate
so, I simply painted both sizes(trailer side and axle mounting plate side) with 3 coats each of POR 15
as I had some on hand
I also went with grade 8 bolts,
only I went with fine thread and not coarse threads as they are less prone to coming loose with vibrations, after they were installed and torqued down good
I hauled a few things on trailer(couple hundred miles or so on some bumpy roads) then went and rechecks torque on them, made sure tire wear was good and things straight as there si some wiggle room for things to run off true!
and once sure, I painted over the bolts again with POR 15, to ensure NO rust and well, its will STOP things from ever walking loose that crap dries like a weld!
its been a few yrs now and nothing has moved and still rock solid
its also the third axle on this trailer, since 1998
If you wanted more exact advice on what GRADE metal bolt to use, you can contact the trailer OEM and ask them, I am sure they would tell you if asking the right person there , just know your trailers spec's well in case they ask, as bolt strength might change from a single place 2500K axle trailer to one that can carry 10k or more!
or even contact who ever's brand axle your going to use
Dexter has always had great customer service from all my talks with them!