Remember that the floor is what connects the seat to the seatbelts, the frame, & the cage. So if the seat rips out, or bends a smooth flat piece of sheet metal into a wave, it means someone's skull is going where it shouldn't. That's why the original floor isn't flat - not even the footwells. There's always a bend or corrugation to make it rigid so it doesn't warp.
So a flat deck over a drawer is probably too weak for a chair that needs to hold a 200-lb guy in a truck going over rocks, even under 5mph. It would have to be VERY thick, and even then, IDK how long it'd stay flat. Even a small channel creating a ridge near each seat mount would help a LOT. See the original ones in those pics above? See how thin the original floor is? It doesn't take much to do a LOT of good.
On Frank's, I went a different route. I built a box that supports a '78-89 passenger seat base so the chair flips up, and IS the lid for the box.
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