Secondary (Winch and other) battery

[Turbogus]

I'm sure there have been many,many posts about batteries for winching and so forth but I've got a unique situation.
'Black Betty' has been running plural batteries ever since I got a winch mounted up five or so years ago. In so doing I pulled an older battery BCI 34/78 from 'BB's stable mate, 'Easy Bake' as being my primary driver I needed it daily for my commute (also dual batteries). The fit was off only by a little bit in the custom tray in the Jeep that I figured I'd address at a later time.
Now that that old 34/78 is starting to wane (not bad for a decade old battery) I'm looking to get a new one but I'm hoping to find a slightly shorter case that'll fit this custom batery tray better. Is there a BCI size no. that is shorter than 10.9" length and 7.8 width?

 

[Turbogus]

I'm leaning toward a 51R at Costco the price is right, still Johnson Controls that is the same I've had in my van for a decade. Still uncertain though.

 

[Lumpy Grits]

Shouldn't both batteries be the same brand and as close to the same age as possible?
LG

 

[Turbogus]

If I could afford two new batteries I'd go ahead, but this secondary battery is only for the winch and a couple of ancilliary circuits.

 

[Turbogus]

It occurs that since I'm also running plural batteries in 'Easy Bake' that are both serviceable, I'm thinking I could sacrifice one of them to 'Black Betty' that is unless my logic is flawed.