I'm definitely not an expert on FE engines but that looks very much like a truck block.
The ribs on the sides and the 4 bolt holes in the motor mount area look like what I remember of the 330MD truck engine and probably the same casting was used for some of the other (361-391) FE truck engines.
I don't know what all the differences would be between the truck and car blocks.
Just another possibiulity.

Richard