It's a shade tree fix, but I've used some painter's stir sticks or sections cut from a wooden yardstick to insulate a battery hold down from cable terminals that come too close to the metal framework. There are also plastic hold-downs available that completely protect against shorts at the cable terminals. A battery that completely fills the tray will have somewhat more cold cranking amperage available, so I'd recommend using the biggest battery you can and doing whatever is needed to prevent a short circuit. A thin piece of wood glued or riveted to the top of the hold-down frame under the terminals would be the simplest, least expensive fix.

The original 6 volt battery was a "Group 1" size. It was almost square, and it just barely fit through the hole in the floorboard.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!