Undercoating & Corrosion ~ 62 Lancer GT

Bea-- have you checked into having it professionally blasted? Just a thought-- it might not cost much more than you buying all the supplies & tools needed.
As for your gas tank-- be sure to save the fuel sending unit from the replacement tank-- also the filler neck has an "O" ring for sealing -- not the doughnut used on later models--- this area is usually crusty, be careful when dropping the tank so you don't break the lip around the tank.
I believe that fuel sender is different than later models & is very tough to find. Newer years are available -- but are slightly different, Lawrence