Bluesteel,
You are probably aware stainless fasteners do not have the same tensile strength as high strength steel fasteners. Stainless bolts are typically just a little stronger than Grade 2 bolts but nowhere near as strong as Grade 5 or 8 and as such should not be substituted in those applications where strength is important.
Not sure which fasteners you are replacing, though if it's to mount the gearbox to the deck, I'd question if stainless would be up to that. Stainless is good in many applications, however when used with non-stainless materials the dissimilar metals can promote corrosion in less noble metals. In most application I use anti-seize when using small SS fasteners to prevent galling.
for more info;
https://www.boltdepot.com/fastener-information/materials-and-grades/materials.aspx