(snip) I built a steel frame out of 1" square tubing and steel plate, around .100" gauge if I recall right, and welded the seat pedestal to that. (snip)
That's pretty much what I did. Being a transit, my bus had a door right at the windshield across from the driver with a pretty steep ramp up to the middle isle. There was also a central "exit door". I decided to build a flat floor from the drivers side over to the other wall, and close in the door. The major frame for the floor I built from 1 X 2" steel and I think my seat plate was somewhere between .100 and 1/8". The area where the ramp was is now an underfloor area (black tank hose? etc.) with a hatch/door to the outside. I want that seat base to be sturdy and safe.