Someone posted a little while back that Odyssey recently switched the polarity of the poles in the 31m-PC2150.
Mine has the positive pole in the same location as the stock battery, closest to the engine.
You can get a 31R-PC2150 but lose the extra threaded set of terminals. You will more that likely need to craft a bus bar to extend the length of the stock terminals. Since I suppose you are fabbing a bus bar anyway you could still get the 31M and put it in terminals forward.
Extreme Series Battery Specifications ? ODYSSEY battery
Anyway just an FYI post.
I would add as long as it keeps the battery from moving during rough driving you should be good. I recall [MENTION=78273]Jeremy556[/MENTION] talking about his scenario running a 31 and his inner fender started cracking, EDIT: may or may not have been from the battery moving. IIRC he wasn't the only one to have this happen they we others too.