Under the hood is the best place to put it. If you don't have room under there the second best place is where the 3rd row seat was. It just means a longer harder more expensive cable run. Under neath were the spare tires was would make the cable run easier but add more problems. You will need to build or buy and mount a full incloser to house the battery. No I have not done it yet but I've been researching it for a while now. I'm designing my own intake for my XK an relocating some stuff and hope to create enough room to put the battery under the hood. I have most the parts I need I refilled my propane tank yesterday so I might just heat the garage up this weekend and tackle that mod.
this has been addressed before a little bit check this link out http://www.xk-forum.com/showthread.p...Second-Battery