Have you considered some type of battery tender from PulseTech or Battery Tender? I use the Battery Tender on my Torino, Mustang, and occasionally my lawn tractor, while the PulseTech keeps my motorcycle battery ready. I used to replace a lot of batteries. Is your alternator factory original? I had a replacement alternator in my Mustang, and it would kill the batteries in only a few months. Turned out the pulley on the alternator was too large, causing the alternator to spin too slow at idle, creating too little voltage/current to maintain a charge. The lights would dim, and the radio turn off when I hit the brakes. My favorite auto electric shop installed a new pulley and it's been fine ever since.