I would at least apply a good coat of zinc chromate on them, to keep them from rusting. It also wouldn't hurt to paint them with a coat of the same color your motor is.
Originals were painted with zinc chromate, then the motor was assembled. Then the whole thing was painted. So, the color coat only got on the exposed surfaces. If you want to pre-paint the mounts and make it better than original---your choice.
Some, repeat, some stainless steel has enough ferrous metal (iron) that it will develop a surface rust. That kind of stainless is often passivated, a process that removes the iron from the surface to prevent that surface rust. IF it were such a grade of stainless, blasting could have exposed more iron to rust. Yeah, I know, a lot of ifs and maybes here. Paint it and forget it.
