I put one in my 18 when I redid her many years ago. Had no problems.
Good peace of mind knowing it will never rot out and spring a leak too.
I actually have a brand new one in the loft at the boatyard that I ordered by mistake.
If it fits, and you're interested I'd be happy to make a deal with you.
There are some pics and measurements in this old thread:
http://www.classicseacraft.com/commu...ad.php?t=24736