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Not sure about your '75, but the later CJs have a removable spacer in the back of that tray which can be removed to accept the larger batteries so you might want to check that.
The marine starting batteries come in basic sizes,
Group 24, Group 27, Group 31.
Other than some odd ball tractor/equipment,
About all heavy equipment uses common sizes,
So we have a ready choice of heavy duty batteries that are NOT particular about how they get charged,
Since tons of industral/farm equipment don't have high performance charging systems, lots of computers, digital injection systems, ect.
Since there will be old construction equipment, older farm equipment, and equipment builders that have common sense,
We will have heavy duty batteries for years.
Torx - My CJ didn't have a battery tray when I got it so a modified 76-86 tray is in it now. From eye only measurements both batteries seem to be the same size.
TeamRush - I was looking at the batteries at Tractor Supply and noticed that they came in some VERY odd sizes .... short and long .... tall and skinny .... long and rectangular .... Very industrial looking sizes as you'd expect to find in trucks, loaders and generators. It is good to hear that they are available in common sizes too.
I's like to run two batteries, but can't for the life of me see where the second one would go. Other than somewhere not under the hood which I'd rather avoid.
'85 CJ-7, 258/4.2L 6cyl. Bought new in 1985. Full cage, Warn 8274 winch, Ford 9" rear, front/rear Detroit Lockers w/4:88 R&P. T-5 tranny and 4:1 t'case. 33X12.50-15 BFG/AT, MSD ign, on board air---
The firewall can't hold the weight and shaking of side-by-side mounted batteries for long with out cracking at the battery bracket mounts.
I have seen this happen many times over the years.
LG
I made a battery bracket out of angle iron,
Added two extra firewall bolts (threaded nut inserts, 'Nutserts', 'Rivnuts')
And added an extra brace under the battery tray.
The batteries sit end-ways to the firewall so the outside battery can come out easily, but that wouldn't matter if you weren't using the second battery for something other than the Jeep.
I tried a couple of the aftermarket stamped steel battery trays and had the same results as LG, so I made my own.
So far, so good...