Sounds like you are a lot like me. About 18-19 years ago, I did a 56 F100 for my dad. Come to find out back then, it had a 272 with heads stamped "ECZ-G".
To find out what you really have in that truck, you will need to get the numbers off of the block and off of the heads. Many times the rebuilders/remanufacturers of engines (big warehouse style companies) that include Y blocks most often mix and match the head and block combo. I purchased an early 60's 292 long block that actually had a set of 1957 ECZ-G heads on it. Nice combo, by the way.
Helpful websites are like the one from Tim McMaster (www.yblockguy.com) and the one that John Mummert has (http://www.ford-y-block.com/). Both of these men have great information that will give you a chart with detailed interpertive information that will help you decode the numbers. The websites also tell you where to look for the numbers on the block (could be one of 3 different places) and on the heads (all kinds of places).
Let us know what you find out. This is great website - you are sure to get info from a good crew of fellas that know much more than I do - I'm pretty much just a Y block nut!
Daniel JessupLancaster, California
aka "The Hot Rod Reverend" 
check out the 1955 Ford Fairlane build at www.hotrodreverend.com