Battary in the boot..

[Conkers]

You can ground it direcly into the chassis/body in the boot so no problem there.
Just be aware of the extra weight (if thats an issue) of a few metres of heavy duty battery cable to run from the positive terminal to the engine bay/starter, alternator etc.
If you are doing it for weight distribution then I doubt it is worth the hassle, and the extra weight will offset any gains... on the other hand if you are doing it to get extra room in the engine bay then I guess you don't have much choice.
If it is going in the boot then a sealed container vented outside of the body is a good idea (a must for competition) alternative is to get a sealed battery or dry cell type.
All depends on your field of useage and your motivation for moving it to the boot.

Conkers.

ps...My car weighed in at around 300kg lighter on the rear axle than the front, so if your motivation is weight distribution then I doubt moving the battery back will make that much difference...