Plonker,
As far as installation I set the Clarke and the Honda tank on the bench next to each other. The two mounts underneath bolted on, the petcock did, and the seat stabilizer screw at the back. It then slid right on the bike and uses the stock cap. Took all of 5 minutes to make the change.
Whatever you get, be sure and flush the tank out before you install it. Mine had some plastic scraps in it.
One more note, I truly believe the Clarke is superior to the IMS, but I got my Clarke for FREE... won it in a drawing from the Suzuki Nat'l Dual Sport Series.
Your fuel strap-on idea is a good one. You may also consider borrowing fuel from other guys on the trip. One way to do this on a 2 smoke is take a quart plastic oil container and put the correct amount of oil in it to make a gallon, then take raw gas a quart at a time from your 4 stroke friends. Fill it 4 times, shake the bike back and forth, and go ride. Be sure and put the first quart in with the oil, not dump the straight oil in the tank.