I'd search around Tacomaworld.com, I think some guys have been able to squeeze a group 31 in the stock location.
FWIW, I have two batteries in the stock location, a group 25 and group 35, which is probably similar to a group 31. I haven't seen much reason to justify bothering with anything more than a stock group 27 there. It's a lot of weight and my fender is slowly sagging. So I'm developing a plan to move the whole mess somehow.