Watch the youtube videos using Harbor Freight aluminum rods w/ a propane torch. They melt ~750 F, whereas pure aluminum is ~1000 F, so more like brazing/soldering and less risk of warpage. I bought some to try when need arises (~$8/pk).

You are correct that welding cast aluminum is more difficult, due to porosity and impurities. I had a crack in my fairly-rare 1964 Valiant manual steering gear-box that a friend repaired with a TIG welder. He had to stop many times to chip out impurities, but the result looks strong.