Do anyone know if a post exists of adding a second battery. I know once you install a RAISS or SSRI intake you have space over the PCM to add a battery, I'm just not sure what all needs to be done to make this work
The battery tray is symmetric and mounts to the inner fender, so you can do just like the diesel trucks of the same era did and put in another tray on the left side, however you will have to re-locate the PCM as there isn't enough height for a 78 series battery. You can probably just rotate the PCM from horizontal to vertical, but will need to figure out a way to fasten it down securely. If running dual batteries, it's highly recommended that they be the same size, manufacturer, and age unless you're running battery isolators and all that jazz.
A 65 series battery will (barely) fit in the stock location if you modify the battery tray - this can deliver incrementally more power, reserve, etc
They have Lithium Ion batteries in automotive sizes that, depending on your application, might be a solution?
As buickman1 asked - what is the motivation for 2 batteries?
Just use the dual battery trays from the 80s diesel B/C/D body cars. Plastic tray in mom's 84 Cadillac, they will fit right in our cars, but you may have to drill mounting holes. Relocate PCM if needed.
None of these mods are for everyone ,or even the average enthusiast. Probably the easiest weight to redistribute. In my car it also provides super stable voltage/ground to the fuel pump. In my truck moving batteries to cabin moved significant weight toward rear by over seven feet. If mod doesn't interest you ,then you probably shouldn't do it. It's not as if underhood temps do batteries any favor either.