Just checked the 1966 Chrysler and Imperial FSM. Only ONE battery size for "all models". Battery designation "27-MB-70". Considering that these models of cars had only B/RB engines, it might be determined that B/RB engine cars came with the Group 27 battery as standard equipment.
On the 1968 Plymouth FSM, which covers all Plymouths, Battery designation "24-MB-59" for non-RB (incl 426 HEMI) engines (all Slant 6s , 273, LA318, and 383). Therefore, it appears that engine size triggered the standard equipment battery choice, although I also seem to recall that the 27-MB-70 battery was an option on vehicles which came with the 24-MB-59 battery as standard equipment? But the 1965 sales guide does not mention ANY optional battery size to upgrade from the Group 24 battery. It does mention that "New for 1965" is a battery with a one-piece top (non-tar top).
As for the Plymouths and Dodges, the bodies did not "know" which engine they would receive when they were put together, so easier to just have ONE battery tray sized for the largest available item.
In the 1968 Fury Fleet "Fast Facts" booklet, it mentions the Group 24 battery as standard on 383s, with the Group 27 battery on all engines larger than 383. In the retail pricing section, it lists both the Chrysler 60A and Leece-Neville 65A alternators also needing the Group 27/70AH battery with them. Which THEN might mean that if factory a/c was ordered, that would need the larger alternator, which would automatically mean the Group 27/70AH battery was in that mix, too. I might also note that the retail pricing pages have "Chrysler Canada LTD" at the top of each page.
A FEW interesting tidbits, too! A '68 Fury 440 4-spd with the "HD SureGrip" option might well be a Dana rear axle AND the AutoTemp a/c was an option in regular Fury V-8 models AND also could be had in station wagons, too. Which then raises the question of if the front ATC also had full control over the rear unit too, or if the rear unit was just along for the ride with its normal non-ATC controls? More research needed, it seems!
Enjoy!
CBODY67