I may attempt to salvage this one first, before replacing it; I'll know
more once I get it off.  The severity of the rust concerns me.  The data
plate you are talking about is nolonger inplace, it already rusted off.
  The bottom of the tank is completed rust covered and pitted up to
about 1/16 inch deep in places.  The tank does hold gas and has no problems.

>
>        If your tank is still holding gas then you may still be able to save
> it.  You should immediately remove the data plate that is welded on at four
> corners.  You will find a great amount of rusting occurring beneath this
> plate.  This is where the tanks rust through if anywhere.  Usually a small
> pinhole at first, then getting larger.  Scrape the rust off the surface under
> the plate and coat with some anti rust paint temporarily until you have the
> time to remove the tank and have it sandblasted and refinished properly.  I
> do them here but it may be a bit far for you to send it here to be done.
>