Quote Originally Posted by Bob Parmenter View Post
Another alternative is a more specific expansion on Dave's comment, take a look at the AGM batteries such as Odyssey. They are very compact relative to power output, can also be mounted in any position, and in my experience more "user friendly" than the Optima. Which is to say they won't die as easy during extended storage as Optimas tend to (yeah, folks have found ways to live with the quirks of Optimas, but why bother?)

If you're going with anything but an ultra high compression, tough to spin up engine the 625 series has shown to crank most typical hot rod engines. The narrow profile of the 625 will tuck up under the dash, behind the back seat, or, if you're set on mounting underneath, will leave more room for plumbing/routing stuff. Even the 925 series (if you just have to have more CCA) is a viable prospect.
I agree with Uncle Bob, the Odyssey/Xtreme style takes the features of AGM technology and packages it in a much more compact, denser package than the spiral wound Optima style which is about the least efficient use of space you can find. There is zero off-gassing, so mounting them in a closed interior area is OK. Some guys, who question the reserve power of the little 625's have mounted two in parallel, but one is good for anything other than a monster high compression engine. You need a charger that's got the voltage limits to maintain the "float" voltage, and not one of your old style high amp, boil the liquid type, but you'll find that the AGM is deep cycle, providing full voltage for longer periods than others, and they can indeed be mounted in any position other than posts straight down.