A 8-D is a huge battery weighing 160 lbs and having around 250 ah. I have one, love the capacity but moving it is heck.
Grp 24 is generally the smallest deep cycle we use, having on average 75 ah or so.
Grp 27 is a bit larger and mine is 95 ah.
Grp 29 and 31 are larger still but not that much more in amp hours and not as much as two grp 24.
6 volt batteries have to be run in pairs, have the same footprint as the grp 24 but taller. They are 200+ ah but take two for 12v.
So two 6 volt will do better than most 12v pairs in amp hours and last longer too.
