I had two 6v Lifelines for my Bigfoot for quite a while that were in the bed of the truck. Lots of capacity but due to the excessive cabling, I had lots of parasitic draw. I traded to a BattleBorn 100AH LiFePo4 battery last year and so far have been pretty happy with it. Yes, it is an investment but their warranty is fantastic, I reduced my weight by ~150lbs and the integrated battery monitoring system ensures that the battery doesn't get drained dangerously low. Something that has already saved me due to the truck being in an indoor storage facility most of the year with no way to check on it.
In regards to your application, I would suggest that before you start looking at modifying your battery compartment to do some shopping around for a LiFePo4 battery that might work. You are going to get more capacity, no hassle with modifying and peace of mind that your battery will keep itself safe.