the original wood floor is a sheet of plywood, well caulked to the body mount flange around the perimeter, not likely any fumes will penetrate it wink it's all about the back door seals, the air flow over/around the truck creates a vacuum right behind it that will allow the interior to "breathe in"

Bill

Moved over to the Passing Lane

"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir
"When we tug a single thing on an old truck, we find it falls off" ~ me
Some TF series details & TF heater pics