For future reference I saw a Group 56 battery in the Sears catalog in its "Plus Start" line. I've heard they discontinued that size in the DieHard line. I purchased my last Eagle battery at O'Reilly's for roughly $100 U.S.
This website
http://www.rtpnet.org/teaa/bcigroup.html shows standard sizes...in millimeters and inches... for just about every 6- and 12-volt automotive battery that's ever been used in North America.
But here in Kansas is a series 56 going to hold to me adding more lights and a bigger stereo? I was just thinking since I'm missing the retaining bolt for the battery that I can Fab a new tray for a bigger battery and have the security of the additional AMPs.
The battery's primary job is to start the engine...in any weather that you would expect to encounter where you live. For this purpose you need Cold Cranking Amps...CCA...and the more the better. CCAs are a function of the battery's internal construction. Kansas winters can be brutal, so one would look for a battery in the size needed that has the most CCAs.
For more lights and big stereos you need a bigger alternator. I believe the standard Eagle alternator is a 55-amp. It's questionable if that's enough for lots of lights and a big stereo. The Eagle alternator is a GM unit, and there are bigger such alternators from GM that will easily fit in the Eagle mounts. There are also other options, some of which need only one wire. You need an alternator with an internal regulator as the original Eagle setup did not regulate the voltage through the onboard computer.