IMHO look before you leap. The floor of my stratos is aluminium, the chassis steel. Sealed (rather than bonded) and rivetted (as wide-headed as I could find) about every four inches, wax-painted inside where I could reach and that black underseal-stuff (?!). The strength is fine, but with time (and therefore scratches, salt, radiator fluid leaks) corrosion starts, and where dissimilar metals touch the corrosion is accelerated (an electrochemical cell, i.e. a battery) - hence the rear floor in the engine bay has given way, the rivet-holes enlarged by corrosion (and the rivet heads broke off). Where they contact, zinc will dissolve in preference to steel, and aluminium in preference to steel and zinc (I think).
Not all is lost - the accelerated corrosion will not occur if water (and contaminants) are excluded. Powder coating will not be sufficient as it does not penetrate tight areas. Applying bitumous (outside) / waxy (inside) materials on all contacting surfaces is paramount to prevent water ingress, but that means you can not subsequently powdercoat. Wet painting is fine, provided it will adhere to the bitumen (some underseals are no problem, but impossible to paint over waxes - use them for sealing after paint).
None of this is going to look pretty, but then how many cars have visible joints? If a rivet or screw is going to show, accept bulbous corrosion protection or bulbous corrosion blisters.
As they say, its all in the preparation.