Could I settile for galvanized steel over stainless for salt corrosion resistance?
I'm a chemist, but this seems more like a question for an engineer. 
Unfortunately I don't have some references to back up my answer.

If you use galvanized steel you have a protective layer on the outside of the steel. As soon as you handle it for attaching, welding... you destroy/scratch/bruise this top layer and the underlying steel gets in contact with the salt solution which is normally aggressive towards metal. This mostly is right for e.g. chloride salts in a mix with oxygen and I'm not quite sure how corrosive MgSO4 is.
Here is a paper about corrosive currents of steel in Saltwater/cement solutions, but that`s not my speciality: http://www.jmst.org/fileup/PDF/pass543.pdf

Stainless steel is more robust, because it's an alloy of steel and chromium. Therefor it's not just covered with a protective layer but saver as a whole. An I guess it's therefor recommended.  

(As a side note, some years ago I had a field trip to a big chemistry company with lots and lots of pipelines. We had some time there with the technical stuff in charge of teh pipelines. They explained, that the most annoying pipelines were the ones with salt water, since it was more aggressive towards the pipes than acids, mineral oils, gases etc.)