Assuming a used tank has been emptied and air-dried completely for a week or
so, what else can be done to make the tank safe to weld?

My local used parts monger tosses out used gas tanks. Three from 2wd Vanagons
this week alone. I was wondering if anyone had ever tried to cut a tank then
lengthen it with parts from another tank to gain extra capacity.

It was just a thought. I am aware that Winkler makes a higher capacity tank.
The price is just a bit disagreeable.

BenT