Your Mustang's battery tray does not have to rust. Here are some suggestions.
It has been more than 100 years since both the automobile and lead-acid storage battery first came into use. Wouldn't it be nice if you could buy a power source for your Mustang that didn't cause battery tray damage? Well - you can. Sealed dry-cell storage batteries are making it easier to both have a reliable power source for your Mustang and not have to worry about acid damage in the process.
Optima makes terrific dry-cell (actually concentrated lead acid, but sealed) batteries for all Mustang generations. Based on personal experiences, these batteries last 7-8 years, never go dead (as long as there's no drain), and never cause battery tray rust. The same can be said for those reproduction Autolite Sta-Ful dry-cell batteries available from Virginia Classic Mustang. Because they are sealed, they don't vent or leak. And because they're designed and manufactured to maintain a constant charge, you never have to worry about them running down.
Reproduction Autolite Sta-Ful dry-cell batteries and all battery mounting hardware is available from Virginia Classic Mustang - www.virginiamustang.com. If you're not concerned with originality, go for the Optima battery at www.optimabatteries.com. Be advised dry-cell batteries don't come cheap - but are worth every penny.