Photos for perspective.
Here is the battery tray. Facing the wheel well you have a chunk of metal that accepts the plastic foot of the battery. On the other side is a bolthole for the clamp that secures the other end down. The battery is further held in place with raised edges.
Now lets add a Group 124R battery.
Again, length is perfect but you can't see it in the photo. It sits in the tray and cannot side back and fourth on hard braking or acceleration. The problem arises that it's oh-so-slightly too wide, so your options are either hook it into the back catch or hook into the clamp but it sits at an angle and not fully in the pan, nor is the bolt hole out far enough for me to still install the clamp and hope the battery doesn't care about sitting on an angle. A hard turn and you run the risk of it rolling out of the pan.
I'll keep poking at this. It's infuriating how annoyingly close this is.
Edited: as a reminder for why I'm needing such a larger battery than the car needs, remember the accessories I'm adding:
-Satellite radio*
-Minidisc head unit*
-Carphone
-MDT 9100 computer terminal*
-Onboard GPS
-300W AC power inverter*
*may be used for extended periods with the engine off
If however someone has a better idea for a battery rating and my solution of just throwing more amps at it is no good, I'm open to suggestions.