PSC put the tank in the bilge on the Mk 2 Orion. The Mk 1, like yours, has the tank in the lazarette correct?
I just replaced my tank this year because i figured after 25 years in the bilge, i'd better replace it before it springs a leak. I have quite a few picture of the replacement job, and i'll post them for you later tonight.
The tank in the bilge holds about 22 gallons. I'll reply again later with pics and details.
BTW, are you a member of the Yahoo Orion Group? If not I highly suggest you join. There are alot of owners, and a ton of great info. I know jknight is a member. Anyway, just follow this link and click the "join this group" button. I regularly post pictures and info of my projects on there. It really is a good resource. I'm SVJessicaAnne
on the group site.
PSC Orion Group