Jim.. you're right.. that seat looks like it was made to go in there!
My idea for whatever seats I end up using in the truck would be simular to your plan of attack.. cutting a sheet of thick plywood to lay over the original fuel tank "riser" and bolting the seat to that, with some metal strips added to the bottom of that board to increase stability.
I'd like to coat that board, along with the rest of the truck floor, with some of that black roll on liner stuff.. that'll help protect the metal, and make it easy to wash out the mud/ice/cow pucky that I track into the cab.
I've really got a jones for a console.. either a fixed console, or a bench seat that has a fold down deal.
