Y-block engine blocks of a given production year may not be the same internally, Tom. IF the block casting was used for making both 292 AND 312 engines, the sand cores used in casting the water jacket SHOULD provide you a block that will take a larger overbore than perhaps a block that was only used for making 272s or 292s that likely used different sand cores in the casting process.
Regardless of the above, Glen's suggestion of having ANY engine block sonic tested before significant overboring is THE thing to do. When the iron enters the casting mold, the sand cores that shape the cylinder walls can easily shift resulting in cylinder walls that are thinner than desired for the boring you wish to have done.
An advantage of the April and after '59 B9AE or F engine blocks is that FoMoco went to longer main cap bolts. Some believe that this was done to reduce a cracking problem that was showing up in the engine block crank webbing area of '54 and later engines that were being remanufactured by Ford or their authorized re-manufacturers. ARP bolts or studs can be installed in the pre-late '59 blocks by drilling, tapping, and line honing the main caps to accomplish the same thing. These bolts OR studs can be obtained from John Mummert at www.ford-y-block.com/
IF your engine block from the '56 Merc is as-originally-installed, it should be a 312 not a 292. All '56 Mercs had 312s.
Block casting letters found on '56 FoMoco 292 and 312 engine blocks
ECK casting letters '55-'56 292 engines
EDB-A . . . . . . . . . . '55-'57 292 engines
ECZ-A . . . . . . . . . . '56 292 & 312 engines
ECZ-B . . . . . . . . . . '56-'57 292 & 312 engines
Block casting letters found on '59 FoMoCo engine blocks
EDB-E . . . . . . . . . . '58-'til April ''59 Ford cars and trucks - 292 engines; and 312 Merc engines
B9AE or F . . . . . . . April '59-'60 Ford cars and trucks - 292 engines; and 312 Merc engines
From John Mummert's web site, http://www.ford-y-block.com/Block%20identification.htm
NOTE: 6015 is Ford's number for an engineblock. All blocks have this number.
Block identification number is generally on side of block above oil filter for blocks cast at Cleveland. Dearborn cast block I.D. number near distributor and above generator. Most Dearborn blocks used in trucks. No Dearborn blocks after 1957.
292 and 312 Blocks were machined from the same castings with the same markings. If the engine block has EBU main bearing caps it is a 292, if it has ECZ main caps it is a 312.
Hope this info. helps. 