G'day if your water pump is mounted on the block then it should be 80mm if mounted to the head then 85mm, BUT the only way to really be sure is to pull the head as if it has been rebuilt at some stage it may have had the bigger bore put into it. If you need rings in one of those girls then you may as well put a complete kit of liners, bearings , pistons etc not much dearer to do it properly.
Jon