what's the ideal battery type/size for a '47 model B?

My manual shows a set of 6v batteries in series. I'm not shooting for a concours restoration application, so a standard 12v battery will do. It's a 12v system otherwise. Just trying to go economical and simple for now. I'm heading over to my fleet supply store this weekend... I'll measure up either a car battery or similar... whatever'll fit and has the most CCA. Just thought there's enough B's and A's (electric start models) out there that there's a "standard" for what's ideal in this application. Anyone have suggestions? Thanks