If you have first hand knowledge of dropping a 31 battery in as the main battery, did you experience and issues with space, hood clearance, battery stability using the OEM hold-down, and were your pos and neg cables able to reach the terminals or did you have to extend the cables?

really depends on what brand of battery you are using, there was some changes when i swapped brands, but also note that how far the post and terminals are.

oem hold down still work, might need longer j hook. i cant really tell you since i changed them out long ago when i have another type of battery, group 48/49 type, they are just as wide, but its 1 inch shorter in height, its used in the euro cars alot.

your cables might need to be extended, the positive side will be snug, the negative side might be tight.

let me know if your on FB , i might still have pictures on my phone, i know @CrimsonCountry might have some pics. check his thread out if you want, we been discussing some battery related things lately.

Yea I just did this a few days ago (see above post)

As ricenadpho said, all the fitment and wire reach questions will depend on exactly which model you buy. Some have the terminals at the front and some, like the marine variants, have the terminals set further forward, too far for the factory connections to reach without modifying.

These wont allow your factory hookups to reach so you'll have to extend them or use a connector like I did. I ended up buying a longer terminal block and it's still tight, but works great. This is without me splitting open the factory loom which would give the wiring a little more slack. I also added 3/8" screw-in terminals in the lug mounts on the block to keep everything stock there.

I avoided the clearance issues as well since I removed the factory posts on the X2 and added the new terminal by bolting it in (if you get the marine version, it's all attached with a busbar under a removeable cap, thanks to riceandpho for that info). The new posts were moved so far up towards the firewall, that even my tall setup clears. It would clear easily if you just used all crimp lugs bolted into the block.

As you can see above, I used the X2Power 31 Marine version. Lmk if you have any questions.