A lot of guys use Group 31 or Group 65. Group 78's can also work. You can make just about anything fit with a little massaging.
The build quality and specs of the battery is the major difference. I would look for batteries around 850 CCA or above. Interstates are very good, but can be a little pricey. Interstates Mega-Tron Plus Group 65 has 875CCA and a 6yr warranty, which is a plus. They go for $135-200. I've run an Interstate in my Grand Marquis for almost 5 years with good results. Only issue was that a cell shorted out 2 years ago, but I took it in and got the battery replaced, no hassle. I think the cell got messed up when my alternator went bad.
Kirkland batteries (Costco brand) are very good, and inexpensive too (under $90). I've had 2 group 65 Kirkland 900cca batts in my 1009 for 3 years now, they've been great to me so far. If I can help it, they will be my go-to battery for every vehicle I own in the future. Downside is you need a Costco membership or know someone who has one.
Guys seem to have mixed results with Optimas; some swear by them, some say they're crap within 2 years. I don't have any experience with them. They are around $200-275.
EverStarts (Walmart) are garbage and you will probably have to replace them every 1.5-2 years (some people say it works out well, because they die before the warranty is up, so you can just go and get a new free battery every couple years)
They range from $80-130 depending on which level.
DieHard batteries (Sears/Kmart) are pretty good. I've had good luck with them in a few different vehicles. They are $115-200 depending on the warranty and style of battery.
AGM batteries, Absorbed Glass Mat, (like Optimas) are made by other manufacturers too, like Interstate, DieHard and Odyssey. AGM's are supposed to work well with higher accessory draw applications, are more vibration resistant, and with vehicles that are plugged in a lot (like with battery tenders which are also helpful in cold weather starting). They also don't need to be mounted vertically, which makes them more adaptable for different mounting situations. But most AGM's are typically in the $200 to 300+ range, and for a vehicle like CUCV's that require 2 batts that can get expensive fast.
The other option is to go with the 6TL batteries which is stock for the CUCV's and other military trucks. Interstate, Hawker, Odyssey and a couple other manufactures make them, but they are pricey ($275-500), harder to find, and weigh double that of a standard automotive battery.
I would look up reviews for the Group size for each manufacturer when you narrow down your search. Inconsistency can happen widely within one manufacturer from battery size to battery size. For example, a few years ago DieHard was ranked 1st in Consumer Reports Battery Tests in the Group 65 category, but were at the bottom of the Group 48 category.
