If I were you, I would get a slightly thicker piece of metal, use silicone to seal it and either rivet or weld the steel in place, and then coat it in tar, or whatever that tough undercoating stuff is, underneath the truck.
Thats what my father did on my 93, the floor had just started to get warm, so we were lucky. He used 2 tubes of silicone, 1 can of the undercoating stuff, a 3/8's piece of steel and a sh!tload of rivets. Pulled the carpet up a while ago to have a look, and it looks new under there still.
When they saftied my 95 they had to fix that hole, and the floor kinda gets warm. They used too thin gauge of metal me thinks
Good luck!