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.