By the time you are reading this, you have probably already resolved the situation; but, for what it's worth, I will stick my two cents in.
I like the Group-31S so well that I have them in every one of the seven trucks on the place; THREE of them in my black Ford.
The 30s and 31s easily fit under the hood of 1980 - 1991 trucks; I can't vouch for later ones.
Another modification that is well worth doing is to get rid of those troublesome lead terminal-ends and replace them with plain old copper "lug" ring-type terminals, then use stud-type batteries, just like the big trucks use.
This is one of the user-friendliest modifications I ever did and I quickly convert any battery-using vehicle I own to the lug terminals and threaded-stud connections.
For those that do not know:
The case measurements of Group-30 and Group-31 batteries are the same; in fact, the only difference is in post location.
The 31s have the posts located along the center; whereas, the 30s have the posts offset closer to one side; otherwise, they are identical.
Plain old Group-30 or -31 have plain old lead post terminals, requiring the troublesome lead clamp ends.
The addition of a letter "s", such as Group-31s, denotes that it is a threaded stud battery with common 3/8 x 16 threaded stud posts.
Also, often, a simple lead post adapter has been screwed onto the threaded studs of a Group-31s battery, thus making it a plain old Group-31.
^^^ Considering the above, if one is in immediate need of batteries, yet the cables have not yet been converted to lug-type terminals, one can buy the threaded stud batteries and add the lead post adapters onto the studs.
Later, when time and money has allowed the conversion to the copper lug terminals on the cables, the post adapters can simply be removed, exposing the threaded studs.
Group-31s is the absolute best no-nonsense choice for a battery; as RSDL? already stated, they can be found in the middle of the night at any truck-stop, any construction equipment dealership or garage, any agricultural concern, anywhere batteries are sold.
As far as what brand to buy, we have at one time or another sold them all, including DEKAs and Interstates, among many many other brands; you will have problems with any of them and no one brand seems to really be better than any other, except the "Neverstarts"---they are pure junk.
Regardless of whose label was on the batteries, the "adjustment" pallet always had plenty of dead soldiers stacked on it waiting when the route truck came by.
No brand is immune from duds.
For the last several years, I have only bought "blems" (factory seconds) for about $38 apiece; no name, no label, 30-day warranty.
I get as good service and performance out of the $38 batteries as I did with any of the $100+ ones that had the sticker still on them.
I hope this information is of some use.