DON'T PLY THE FLOOR !!!!!!!!
When they do they will screw throughe the floor thus making it easy for the rust to start around the screw holes.....
I have got Speedlining on the floor and ply on the sides with the Speedlining up to the wheel arches, When fixing the tank to the chassie you must put the Speedling/Protectorcote around the holes with some anti rust stuff thus making it rust proff and in five years time not making you floor drop out....