I'm planning on a couple of 24-gallon saddle tanks by Moeller for my boat project. This model is fairly wide however and would protrude into the cockpit area a few inches past the six-inch wide gunwale.
Greg's old Web sites were on MSN and they decided to no longer be a host. They're apparently just gone. Greg's tanks also protruded into the cockpit area; I'm not certain of the brand and model he installed but in my recollection of his photos they showed a tank of about the same size as the Moeller 24-gallon saddle tanks.
On my boat, I'm planning on redoing the gunwales anyway, so I'll widen them to enable coverage of the tanks on the top. Since they're 'below decks' tanks they'll also be hidden by fiberglass covers but for a window facing forward to allow a visual check of the levels.
Tom