Leaky Holley Main Body (porosity?)

Looks to me like whoever converted it to 4 corner idle broke through when making the connecting passage. If you really need full four corner idle, then its better to start with a casting with more meat there. A -2 might have it whereas a -4 may not. You'll have to study them to know. Can't help further on that main body. I think I have a 4779 but like yours the choke horn is cut off so the revision is unknown.