They have a bunch of different configurations so you should be able to find something that will fit. As TheDancerII said you need to have 2 of the exact same battery and also both should be new otherwise the older battery will be parasitic on the new battery having a lower capacity from age.
I installed Northstar AGM's last year. They are amazing. They are about double the price but they last 3 times as long as lead acid. The estimates are 10-12 years so if you plan on keeping the boat AGM is the way to go. Just verify your charger is compatible with AGM chemistry. I went from one battery to two 180 amp/hr batteries and they are almost the same size as one 8D. They have standard sizes but also have slim designs like a 4D and smaller. No affiliation to Northstar other than a good experience this year. There are many other good companies but a bunch of bad ones so do your homework.
One thing to watch is your the weight. Adding a bunch of weight to one side could give you a list or worsen one that already exists. I added all this weight because I was trying to remove a list which worked out for me.
I also installed a cheap digital voltage gauge in the cabin to monitor the battery. It worked great. There are voltage to life charts online to help from going below 50% capacity.
_________________ --2002 3880 Regal Flybridge | Twin Merc 8.1HO **Seven** --2003 AB 13 VST Console Inflatable | Tohatsu 40hp TLDI **Phish'n ski** --310 Mercury Inflatable | 1982 9.9 Johnson
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