Don, I guess I'll make the highjacking of Youther's thread complete. First off, I'm not big on rumble seats, so I'll just throw out some things to think about. Of course it's your car so you choose. Putting the tank and batt behind the seat pretty much eliminates any leg room for the rumble seat area, and that in turn means you have zip for luggage either. Small kids could sit on their legs for these parade moments you're contemplating, but then you also have the issue of them getting in and out without harming your paint. All in all, in my mind anyway, the notion of the rumble seat is nicer than the reality of it.

One way to gain some batt space is to use an Odyssey Battery. Foot print is roughly 6.5 x 7 inches for a 925ah batt, at under 6" high. They're dry cell and can be mounted in any direction. A tad spendy compared to "normal" batts at your local auto parts place, but a real space saver, and easier to pull out if necessary. As you already are aware, under the floor there's precious little space with dual exhaust, so minimizing the above floor loss is important.

Below is a pic of the master cylinder access in my roadster just to give you an idea. Simple piece of sheet stock with a short "piano" hinge spot welded. You may have to beat your upholsterer around the head and shoulders so that he doesn't glue down your carpet as they are wont to do!