Rusty floor pan!

My first comment is we really need some pics to know whether the patch is even sufficient. Regardless which method you go with you need to remove any rot and avoid overlap that can trap moisture or it will eventually rust worse. Seal both sides after it is structurally sound.
I think all the 'right' ways are covered pretty good here. No doubt that welding it is the best, adhesives are probably good too, but are even more dependent on good prep and fit than welding.
That said... in my youngerdays, I metal taped, riveted in any old scrap of metal I could find and did all kinds of makeshift repairs just to get through inspections and keep beaters on the road. None of them ever fell apart, but I was not building high torque race machines either. No I am not suggesting that you do a cheesy repair!
Back to the original post If they used aluminum rivets, they would react with the steel over time. Steel rivets are a 'structural' fastener. Combine them with any good metal epoxy and you can fix this pretty solid. Again I am saying this could work for a few small repairs not suggesting it a 'good' way to build a car.
So, before you even start in like that. Where are you going with the build? Is there other stuff that should be welded? I know I asked myself that when I started mine and the clear answer was - Quit messin around and get a welder. Best decision I made...