I went to my local Ford dealer and purchased a Group 24 Motocraft battery. I have this battery in all of my Mustangs and they hold a charge forever!
For shows, I took a repro Autolite Groupe 24 battery that went dead and took out all of the acid and lead plates. This is quite easy to do (be careful with the acid and put it into a plastic container) by taking a die grinder and cutting along the top seam of the battery. Once cut, the top of the battery can be removed, exposing all of the lead plates. The lead plates can be recycled. Wash out the battey case with baking soda and let dry. Once dried, use black RTV and glue the upper case to the lower case, and now you have a "show battery" that weighs 2 pounds!! You will need to change your battery at the show, and your lights won't work, but I found this to be an economical way to have an Autolite battery at the shows. I just keep the show battery and show brackets in a box in the trunk.
Hope this helps, Russ