Don't let their paranoia stop you.
It's not the tank........it's the fumes that you have to deal with.
Unless it is rust they are worried about , there is no valid reason not to if the proper prep is done.
I did a dukes of hazard thing in a 1963 sport fury one time , we pulled the tank outside a gas station in the driveway with tools loaned us by the attendant. Took it to the welder that did my dads farm equip. welding, he was asleep in a chair in his overalls at 2 in the morning. He washed the inside of the tank with carbon tetrachloride and welded it up , no problem. Fast forward yrs. later, I was building a tank for a fully tubbed hemi belevedere, out of an old s10 tank, couldn`t get carbon tet by that time thanks to epa, washed it w. water / soap and everything we could think of, left it full of water , and it still blurped when I put the flame to it. The tanks had very high lead content back then, and could be soldered instead of welding. I have then since inquired about getting a sump put in, and no welder wants to do it !
LOOK INTO DEISEL ADD ON SUMPS IF U NEED TO GO THAT ROUTE.
