I don't know a lot about this, but I have heard that there were casting porosity problems witha a lot of the aluminum manifolds, so you might be better off using a cast iron manifold. Probably a lot easier to find, too...
Additionally, those OEM Weber carbs for the Pintos, etc. were on 140 cubic inch, 4-cylinder engines (unless it was for a V6) and probably aren't sized correctly for a 225 slant 6. Holley makes a 350 cfm 2bbl that I THINK is based on the Weber design, that would maybe be a better choice.
Dan???