I have a buddy Jody who has a site weldingtipsandtricks.com who has a video on how he did an aluminum intake manifold. Like cast iron, sometime aluminum takes some preheating to make it a good weld.
Follow the link and watch his procedure on this type of repair if you're a welding enthusiast. Jody is a true pro and offers a great volume of free videos on welding should someone in the forums be contemplating this.
Three parts including Tig... watch all of them
I think the challenge may be in the aluminum alloy mix MAGNA used in the 1940's early 1950's compared to what we do today.
http://www.weldingtipsandtricks.com/tig ... ntake.html