The ECB-B four barrel intake was originally designed for the 1955 models. It’s easily recognizable by the vertical flat on the right side of the intake for the automatic choke hardware. The '56 models had the automatic choke hardware moved up to the carburetors. Here’s the link to a past thread giving some of the casting numbers for the more common four barrel Y intake manifolds.

http://forums.y-blocksforever.com/FindPost37008.aspx