How Do I Charge, and Maintain 2 New 6 Volt Batteries???

Exactly. 27's are basically almost the same amp hours (up to 230 for a pair) and 31's are about the same size (longer but not as tall) for comparable AH, but more money. GC 6v batteries are cheap and tough for the amp hours you get, but not really more amp hours except compared to tiny group 24 batteries which are physically a lot smaller.

Jim