pull of the flywheel pan and check the crank flange.the 3.3 crank 272 and 292 will have a half moon cutout and opposite will have a recess cut out of the crank.its about an inch long and not very deep the 256 crank will have the half moon only.

The Casting # EBY is 1954 Mercury 256. Looks like someone swapped Motor and installed 272 Heads. You can get more ID Information by going to John Mummerts Site using the "Links" at top of Page.

  

I acquired a 56 F100 frame with a Y block engine. Decoding the frame VIN it indicates the truck was equipped with a 272. 2V. The engine block is cast with EBY (256) the heads are cast with ECZ (272). Pulled the distributor and it has 14 tooth gear and hex drive. Is this a 272 engine that possibly cam with truck or down the line did someone put 272 heads on a 256 block. What else can I do ie drop oil pan, pull cylinder head to help verify what I have?