D@vid -- make sure you're getting AGM-type batteries for your T (which will minimize leakage possibilities) == there have been a number of threads about alternate solutions (including using Miata batteries) - see, e.g.
http://forums.pelicanparts.com/porsche-911-technical-forum/564510-early-911-battery-options.html -- Amperage-wise, even a VERY small battery (like a wheelchair battery) will start a 911, amperage ratings on car batteries go more to reserve power (i.e. ability to still start car if you leave lights on overnight) --