My old bonnie has started to showing up multiple floor perforations caused by severe corrosion.
I would like to know what are the options to reseal the under body of the floor and restore the floor body strenght at the same time. (i.e.: I am not too hot to just fill the rotten holes with black pitch)
Is there anybody here who has done a serious body fix of this type?
I was thinking to remove the seats and carpet and double the floor with sheet metal plates from the interior.
I am wondering if there a way to restore the floor also from the outside side of the floor?
Welding them or rivet the plates?? I am not sure where to continue at that point.
Any comments will be appreciated.

I've never heard of the floors rusting out like that on these things, at least not without the rest of the car rusting in unsafe quantities. Are you sure the car is still structurally good/safe?

Years ago, I repaired an old Mustang by using the 1/8" hard aluminum from old street signs. I bent and shaped them to a fair simulation of the inside body pan, treated the rust with rust converting liquid, used black jack roof tar between the panels and riveted the new panel in from the inside.

I then rust treated the outside/underside too and used a heavy undercoating on it all.

Now keep in mind that the method I describe above is more of a shorter-term patch than it is a permanent repair, IMO. The one I did lasted at least 4 years until I sold the car and that was in New York. A more permanent repair would be to cut out all the rusted area and replace with new metal, but that is very labor intensive and more costly besides.

Your call.

I did a similar repair to Lash on an old Buick Skylark I had. Only I used some sheet metal from the side of an old Washing Machine we were chucking out.
The correct way to do it IMO is to weld the panels in. then apply sealer and undercoat.

I agree that that is a better way to do it, but just wanted to point out that the method you use depends upon what your goal is to start with. One technique is used if you are only trying to patch something for a few years until it dies; and another is better, more permanent, and lasts as long as needed.

I appreciate your comment and of course and you are right, this is the first thing to consider before attacking a fix of this kind.
I will have it inspected for safety before getting any further.

Thats the point as lash stated. How long you want it to last. It would last well with rivets but would not meet some local inspection codes.

Inspection, there is not much of this applied in the Province of Quebec compared to Ontario or many US states where you are obliged to pass an yearly inspection, so I will not be too much bothered about this type of control.
On the other hand, well... the best years of my car are over for sure but this little hell is just don't want to give up!!! The softness of the floor is an issue I need to solve to make sure it is safe thow.
Someone has recommended at work the following repair protocole;
use 2 sheets of sheet metal (gauge 16 or 20) sandwiching the current floor in between and secure the new sheets by drilling several thru holes to suit steel corrosion resistant bolts, large washers and install 2 nuts per bolt to interlock and torque each nuts at 65 lbs.
Once this is in place, weld the newsheet metal plates to the rockers frame.
The only thing that he state thowis that the rockers under each doors must be in good condition (not rotten by corrosion) and if they are bad, that I must fix them using "all metal" filling to restore the rockers strenght.
After that, coat inside out with a primer and an anti rust enamel.
Is it making sens to you guys?

I've never done this, but sandwiching seems a bit excessive, especially if you're welding anyway. If you're welding new panels in, you might as well just cut out the old, and weld in one new one. Seems like a single sheet repair would do just fine, either top or bottom.

But as I said, I have no personal experience to back that up.