Our 2xGrp 31 AGMs are secured in essentially the same manner as Steve's, and we've never had any issues.
It sounds like your surveyor made the PO fix something that wasn't broken or hazardous, but the "fix" is. Also, I don't think the P.O.'s switch to GRP 31 would have altered the original set-up in any way. Our boat originally had 2xGrp 27s, but no change was necessary for the Grp 31s -- they are for all practical purposes the same dimensions (but different weight.) In fact Grp 27s and 31s use the same size plastic battery box.
As for adding the third starting battery -- I've wanted to do that too for a long time but have never got around to it. If I did add one, I've concluded I'd put it on that little shelf just above and aft of the stuffing box. Others have put it there, too. Here is where an AGM or similar would be best, since it could be installed on its side which would allow much easier access to the posts for cable attachments.
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Pacific Seacraft Crealock 31 #62
NEVER CALLS CRUISINGDAD BACK....CAN"T TAKE THE ACCENT