You'll have to elicit a response from someone who's tried (I'm still operating on the original batteries w/ my 2017) but I think it's possible to squeeze Group 27s in the current battery box. I'm hoping so!
At any rate, the trade-off between 6v-series and 12v-parallel pairs is that the 6v batteries are a bit "tougher" and tend to have more amp-hours at the same physical size, but if you lose 1 6v to a failure you've got zero battery power, whereas if you lose one of the 12v
pair AND catch it before it takes down the other battery in the array you can theoretically limp along at half capacity until you can replace the batteries.
Lifeline's GPL-4CT is close to the size of a group 24 (it's 3/4 of an inch taller and a half inch wider, but not as long) but they cost 2x what a wet-cell group24 12v
does, and $40-$50 more than a Group 24 12v Lifeline, but at least in theory you get about 50% more amp-hour capacity with the 6v pair in series.