Before you get too far into this, be sure to take into account the price for all new battery cables. It is HIGHLY advised to use one continual length of battery cable. Also, since you're going a further distance (although I'm not sure how much further, off hand), you'll need to get larger ($$) cable. Good quality, tinned, marine battery cable is not cheap.
You might want to experiment with simply putting some extra weight where you anticipate putting the batteries and see how much of a difference it makes.
As far as actually doing it, there's nothing too special here. Run the new cables, support/protect them along the way, add new fittings at each end, re-mount the battery hold-downs and you're done. I'm making it sound easier than it is, but the best thing you can do is just get your head in there and look to see what you're dealing with and where you will run the cables.