[wilf]

I echo tarmacscratcher's comments.

Most stainless "dress up" fasteners you can buy are of very poor tensile strength compared to normal hiigh tensile bolts. In addition, stainless fasteners are prone to "galling" (seizing up to you and me!)

Whilst there are grades available that would be suitable, to understand them, identify them, and track them down is a task for an expert. Try a search here on "stainless" or similar, there has been good info posted in the past.

Of course, you might decide to become such an expert, but personally, wherever a fastener is safety critical, like suspension mountings, propshaft couplings, engine mounts, safety belt mounts, etc., I would just use an 8.8 or better high tensile normal steel nut & bolt.

Edit - Oh, and what Purple said too.