I have been lurking in the back ground learning what I can and now starting work on a 1957 century with a interceptor marine 312 y block the interceptor model number is 215 the ser num is 35596 on the rear of the block of the rh bank just under the head is stamped 312 35556 & on the left side of the block over the oil filter the block num ECZ - 8015 - C. The marks beside the disturbutor port shows to be a Cleveland. The number on the heads are ED - ECZ-C with a 1.78 valve, this is were I get confused from what I read the heads are from a 1956 truck 272 cu.in. To my knowledge this is the original and has not been altered, although it appears that one of the heads have been taken off at sometime.

Preserving the old can be more rewarding than buying the new...

Sorry after I posteb photo I could nut add more to it.I am in need of the flame arrester air intake to the carbs as in last pic. The later model are open with a brass grill on the end which I would prefer.
Thanks for your help Ike

Preserving the old can be more rewarding than buying the new...

I can only guess at what the rules were for the heads that were supplied for the marine engines.  Where the cylinder heads for the cars and trucks are pretty much defined either by year or cubic inch, the marine engines would appear to be more dictated by the required horsepower or torque.  As such, it’s possible that the 312 engines simply received a smaller valve head to get the torque numbers higher or to actually limit the rpm capability.  I’ve dealt with several 312 marine engines and the cylinder head choices for those seem to be all over the map as the intake valve sizes are varied.
 
Here’s the link to a recent thread showing some pictures of the marine valley covers.  I suspect your cover has the air cleaner built into it under the intake manifold.  Those particular covers are missing the fuel arrester bonnets quite frequently when they arrive here.
http://forums.y-blocksforever.com/FindPost116597.aspx

Lorena, Texas (South of Waco)