As Dewey says, it is a good idea to get hold of a genuinely scrap Douglas casing to practice on, before doing further damage to yours.
Now, a bit of advice that may help. When I needed to have some welding done on postwar crankcases that had previously been gas welded, most of the welders that I contacted said that it couldn't be TIG welded as there would now be too many impurities in the metal. At the time, it was easier to find a pair of unmolested cases and start again. Later, when I built the four cylinder special, I entrusted the work to a welder from the aircraft industry - the finished job was superb - after fettling, the welds were invisible, so I took the opportunity to pick his brains regarding the other cases being unweldable. He just smiled and said "What you have to do, is adjust the set to AC with a greater bias toward the cleaning phase, then reduce the current so that it does not weld - make a few passes over the area to preclean it (a bit of brushing with a stainless wire brush helps to remove the impurities - don't use an ordinary wire brush, that contaminates the area again) - then readjust the set and do the welding as normal". I tried it at home, and it works!!