Take an Autolite battery box, cut the top off and remove the insides carefully. Get a wheelchair battery, set up the posts with shorter cables in the box, and you're good to go. I've had that in my 68 coupe for more than 10 years now. If you keep it on the battery tender the small batteries will last three or four years. And, they start my 289 just fine.
It is harder with the 65/66 models, but it can be done. My friend did it on his Shelby.