Erica, the original I belive was flat floor and on occasion it raced using a side exit exhaust, to achieve this it is necessary to have at least the LH side flat floor, or the exhaust would be lower by the amount of dish in the floor.
Whilst I have no intention of outright racing the completed car, (perhaps a couple of track days) someone in the future, when I am long gone, may wish to do so.
My wife accompanied me on many E Type trips, but I suspect will be reluctant to do so in the Low Drag given its lack of creature comforts. Its missing even the basics of heater, radio, noise deadening and only has one wiper positioned for the driver. Given this plus the side exit opportunity I went for the LH flat floor option.