The old 911s use a super-short ground cable. It just goes from the battery to the chassis, right behind the headlight. Super easy to disconnect.
Then it's just a matter of making a new cable of the appropriate length. I decided to attach a battery tender quick disconnect while I was in there.
I then cut the head off the positive cable, attached the new head, and added all the little fiddly miscellaneous wires that always seem to be attached to the positive lead. (Plus one wire for the tender). Sat the battery back down in the hole, connected everything up, and heard the clock inside start ticking. Looks like it works.
Onwards!