For concours judging purposes, yes, '57 'C', 'D', and 'E' code 'Birds came equipped with LARGE letter ECZ-G cylinder heads. The point here is that there are two basic series of ECZ-G cylinder heads. The casting letters are the same, but the letter size and font for the ECZ-G letters are easily told apart.
The two series were cast in different foundries and to my inspection while the combustion chambers appear to have been formed using the same sand cores, the sand cores used for shaping the intake and exhaust porting were quite different.
In casual outward appearance, both series of 'G' heads appear to be identical. However, to anyone who knows 'Birds the casting letters are easily seen, easily told apart, and are looked for in determining originality.
As you get into this judging stuff, I THINK you'll find that later Ford replacement parts for original issue components will not cost you points. IF that's true, in subsequent years the 5752-113 cylinder heads were listed as replacement heads for the ECZ-G units.
Click the link below for a thread with pictures, description, and letter orientation information to help you in telling the large and small letter 'G' heads apart.http://forums.y-blocksforever.com/Topic113734.aspx#bm113748
In mentioning intake manifolds, yes, you want the '57 'B' style for your 312. Interestingly, '57 Ford 312 engines came with the first year (new) Carter AFBs, Ford 4100 shoebox carburetors, or Holley 4150 series end bowl carburetors. However, '57 'D' code 'Birds were only produced with the Holley 4150s.
Hope this helps.
a.k.a. Charlie Brown
near Syracuse, New York