I did alum floor pans. But not to fix a rust problem. Did it to lower the seating, interior trans x-member support, and increase overall 3D structure. Yes very labor intensive indeed. The strength in most surface situations is much stronger than welding to thin sheet metal. Ferrous and nonferrous metal is not a problem. I have used alum all over the car in many places for many years now with zero corrosoin problems. It's really not an issue. Pics in my hybrid galley show many areas of use. (Seat pans, fuel box, sub frame connectors, rear hatch tailgate support.)
http://album.hybridz.org/showgallery.php?cat=500&page=3&ppuser=10889
For your rust problem though. I would weld in some steel new floors. You could riv in alum and seal it up and it would work great. Though most people looking at your car for resale would think it was a hack job when it's not. People are stupid sometimes, like welding is king or something. Too much pimp my ride tv. If a Z was build using rivets instead of spotwelds just think of the cost factor. Yes the weight but most weld stitch there car too so go figure the real weight. Too exotic. Built like a real aircraft, like a WWII Jap Zero. The abilty to maintain/replace joints and substructures and spreading out the load in joints, far superior. Ok guys don't kill me now. Each process has its own positive and negatives sides. I just see many doing restoration falling into "I have to weld it" when other alternatives could be implemented with better success.