Stolly,
I fitted a heavy duty battery to my R33 GTR a month or so ago, and despite looking a little tricky its an easy job.
Simply remove the single bolt on each side holding the rear brace in, then remove the battery clamp and prepare for a good half hour of leaning over and loosening off the battery terminals, as the others have said you need to use some care here, but essentially its a simply swap - out with the old, in with the new.