What one finds in dealing with old cars and corrosion is that many times you can not really see how bad the metal is... so when you start to weld.. either oxy acet, stick, mig, Tig, etc... once you put the heat to what you thought was good metal to attach TO.. just literally MELTS away... not there anymore...
So usually best to start with a method which will preserve the strength and ' existance' of the metal you need for support... and just follow the ' rules' , in this case sheet metal rivet rules', and then seal up from further corrosion the best you are able... but that process also must follow the rules... as you do not want to encapsulate moisture in a pocket where it will be working away like termites on your structure...

and remember,,, there are lots of legit pop rivet uses... but here we are talking about SOLID STEEL..... rivets for this structural repair. Everyone ought to have a paper copy of that aircraft tools catalog... very interesting reading... and cool tools.. even though many of those tools will be available at less cost elsewhere.....

The use of Clecos.....
https://www.youtube.com/watch?v=aTQNuQlVnA0

So these are very useful all the time... but particularly when replacing a full floor pan.. you can use a few of these removable fasteners to get your floor pan positioned and measured at exactly where you need it... floor pans are hard to work on due to not being able to put your weight on them... so a few holes of the same size Clecos you have... then tweak the position.... and put more in... until you are sure you have it where you want it.... you already have flanged the sheet metal which is going together...and made plug welding holes probably.... or brazing holes... interspaced between the Cleco holes... then you take out the last of the Clecos and put in your final solid rivets......and then prep for sealing it all up....

Great Sheet Metal automotive books... look for author Ron Fournier a magician with sheet metal...
https://www.youtube.com/watch?v=aTQNuQlVnA0

Either method of repair will work.. just be sure to cut back to solid material... clean and prep....

And be ready to go back over everything with a rust preventive. Even drilling holes for rivets exposes the material to potential corrosion. Clean, Prep, Repair, then protect.