Manifold and carb question

Some number of those cast iron intakes have been weld repaired at that burn through.
It may not be a very common failure but yours isn't the first.

Those aftermarket manifolds are made for 4 brl carbs and have larger ports to match 4 brl 340/360 heads too. You would need to change a bunch of surrounding parts, air cleaner, kickdown linkage, etc..