Bob, I have been involved with hot-dip galvanizing of structural steel sections, but never thin-section base metal such as a fuel tank. Nor can I recall the % make-up of material used. Therefore, can't comment with any direct knowledge, but my thoughts are:
- the process may distort the shape of the tank.
- the existing holes in the tank (such as filler neck and sender hole) while large diameter, may not be best positioned to allow fast draining of the zinc on removal from the tank.
- the finish may not be suitable to you from a visual point of view, being an industrial coating as opposed to an architectural finish.
Having said that, I believe the reproduction tanks (such as those made by Spectra) have a gal finish, but not sure of the process used; it may be that they are produced from a cold-galvanized material.....
Perhaps best to contact a local company that does this process and discuss?