Now I understand...
The H3 does have a tray with a plastic cover, with a small tube for cooling that feeds air into the box, by the front grille.
From my measurements yesterday, it does appear the plastic fins at the bottom of the tray would need to be cut (Dremel time). These fins are used for reinforcement, and only a small amount on each side should do.
I would suggest if anyone is adding an Optima to an H3 to remove the battery and do some measurements against the Optima battery size specs.
As for the Optima battery...nothing is better. I carry an optima in the back when off-roading, charge it just before the trip. You can actually turn the Optima upside down and touch battery terminals to start a vehicle; so long as they line up ok.