I use either group 26 for a more square but shorter battery and group 51 for a more rectangle shape (1" less on width) that hugs the frame better.
S12V925 series battery is perfect for any hot rod. Turbostart also offers dimple die mount or billet(yuck) mount. I saw one guy say he uses a 680 series battery to start his flathead. Perfect choice for any coupe or roadster with a low amp alternator(70 or less). Any higher could swell the small battery on long cruises. The 925 will work perfectly and can be mounted in any configuration.