Hello blackdragon, if I were you I'd go with the Galv option its practical.
A stainless chassis isn't impossible but its highly impractical, trust me I'm an apprenticeship served Fabricator / Welder with 27 years under me belt. The cost would be astronomical not to mention the cost of specialist labour and the jig. Apart from these factors more and importantly, the vehicle would need to be subject to a Single Vehicle Approval test if the stainless option were pursued because the chassis would not be classed as a "direct replacement".
I'd relish the idea of a stainless chassis
Regards WP