The engine serial numbers were normally stamped on the machined pad next to the distributor.
I don't think there is any way to thoroughly clean the gas tank without removing it. The chemical cleaners made for this purpose require the tank to be placed in various positions so that the liquid can cover every square inch of surface. If you go through the trouble of cleaning, you may as well seal the tank, too. See
here. Alternatively, you can buy a new (or reconditioned) tank. They are available at most of the truck parts vendors (check the Stovebolt links page). Price range varies.
