On my 55 FC the floor width was 90". The floor overhangs the outriggers slightly, about enough to keep the ends from rubbing through the bellypan (hopefully) It is my observation that all of the floor widths I can recall were whole inch measurements. if I am wrong surely someone will call me out but for initial planning at least I would measure the outrigger width and you will probably be 1/4 to 1/2 inch shy of a whole inch width.
If your frame is anything like mine you will probably also find that the outriggers vary slightly in length. Mine have nearly 1/2" variance from short to long with the longest actually being hammered shorter by someone at the factory to make it fit- awesome QC