Don't forget, you backseat battery folks, that the battery needs to be vented
to the outside of the vehicle to prevent the accumulation of explosive hydrogen
gas. The stock batteries come sealed with a vent connection. 5000's actually
have a vent tube routed to the outside of the car. There are many aftermarket
batteries that will fit, and have sufficient capacity, but may not have the
vent connection. Make sure to get one with a vent connection; they are
available, and safer.
Of course, none of this matters if your battery is located under the hood.
p.s. I wonder if the old VW Carmann Ghia's had vented batteries? Maybe they
just blew up.