Optima battery??..

j par.
Battery in trunk means you should use some sort of 'sealed' and probably a gel cell or AGM battery unless there is a dedicated box and vent to outside.
Optima is just one of many manufacturers of such batteries.

It's unlikely the car's equipment will draw down the battery like one would do when trolling on electric power in a boat. Your ammeter will clue you in to whether this is the case. No ammeter? Then assume for now that's not the case because you've said nothing about trouble starting or eating batteries. I know for drag racing some people will purposely run on battery only. So there, maybe. Otherwise, a battery for starting is the important thing.

Anybody running the AGM in the Mopar style case?
They recommend charging on 10 or 12 amps only, every two months during storage.
Seems to have really held a charge (over 12v) on the shelf for entire winter.
1965-1974 Plymouth Barracuda Parts | MD2467 | 1965-74 Mopar A/B/E-Body AGM Goup 24 Maintenance Free Battery - w/ Yellow Caps | Classic Industries
View attachment 1715549620
What we are reading on Classic's website has a big red flag that its marketing BS written by someone who has no idea.
AGM is a battery made with liquid acid in an Absorbant Glass Mat
Gel-Cell is, well, an acid in gel form.

As far as the instructions go, its a little hard to know quite what they mean.
It says charge the battery to maintain 12.84 Volts. That's different than charging at 12.84 Volts.
Recommending a charger that is rated for 10 or 12 amps is not really informative.
To reverse the chemical reaction that occurs during discharge, its neccessary to connect to a power source at a slightly higher voltage than the battery.
To force 10 or 12 amps when a battery is close to fully charged will require supplying power at a high voltage. Perhaps they mean an automatic charger that is capable of 10 to 12 amps, but will reduce as the battery charge increases.

All car batteries that we use should be around 12.8 Volts when fully charged and in good condition.

--------------------------------------------------------------------------------------------
Cranking Battery vs. Deep Cycle Marine Battery
Wayback Machine see especially sec 7.1.8 for deep cycle vs starting, and 7.1.7 for gel cell