Choosing a BatteryFor vintage Mustangs, choosing a battery for a regularly-driven car isn't too difficult. The most popular size battery for vintage Mustangs is a Group 24. Most small-block V-8-powered cars with 260, 289, 302, and 351W/C engines came with a Group 24-sized battery.
When big-block Mustang production started in 1967, they were mainly equipped with dimensionally larger Group 27 batteries. Thus any '681/2-'70 428 Cobra Jet car was equipped with a Group 27 battery. Anything larger was relatively unusual in the realm of early Mustangs. The huge Group 29 battery mounted in the trunk on '69-'70 Boss 429s is the most notable exception.
Most of the time when shopping at an auto parts store, you'll be steered to a Group 24 battery for almost all small- and big-block V-8 Mustangs, which usually works fine since the cranking amperage for modern batteries is considerably better than earlier batteries, regardless of size. For example, the Group 24 Autolite battery we purchased at a Kragen Auto Parts for our '69 Mustang project car has more cranking amps (835) than any Group 27 battery sold in 1969.
For a restored show car where appearance is important due to show judging, Antique Auto Battery offers raised-letter batteries that have hard-rubber script cases and correct Autolite red caps. AAB also has an epoxy-type material called Poly Tar for its reproduction tar-top batteries. It duplicates the appearance of the old-style tar material that got sticky, dirty, and sometimes leaked acid, but without these obviously undesirable qualities. For vintage Mustangs, AAB essentially has all the cars covered with G24FA, G27FA, and G29HR sizes.