Just my 2 cents worth on this topic. There are only three major manufacturers of batteries in the U.S., Johnson Controls, Exide and one other I can’t remember at this time. They make most of the batteries and just re-brand them for most everyone else.
There are only two companies that make lead acid 6TL batteries that are Mil-Spec, Exide and Deka (East Penn Manufacturing Company, Inc.)The Mil-Spec batteries have heavier plates, stronger cases and deeper sumps so they don’t sulphate out as fast as a non Mil-Spec. battery.
Personally I prefer the Deka battery over the Exide battery because it has 750 CCA at 0o and 230 minutes reserve compared to 600 CCA and 200 minutes reserve.
There are pluses and minuses with using 6TLs compared to say a group 31 battery.
The 6TLs, if taken care of will last 8-10 years. I just replaced some that were made in April of 1999. They fit the battery box without modification and the cables with fit.
However they do cost more, I just picked up twelve in Baltimore the other week and they cost $135.00 each. These are first run new production batteries.
Other batteries may be cheaper, group 31s are around $65-$80 in this area, however they with probably only last 4-5 years. They will also require some “rigging” to hold them in the battery box. They will probably be easier to find.
For my money I think the 6TLs are worth the effort since I don’t like handling those things any more that I have too.
There may be a Deka dealer in your area. The Napa dealers in this area also carry 6TLs and maybe they can get them for you.