This was copied from an article Written by Barry Robotnik - who often visits this site - on the following website
DIY Ford - Do It Yourself Ford Projects.
The “Drill Bit Test”
This one test is the single best way to quickly identify an assembled FE block, and credit for it goes to FordFE.com forum member David “Shoe” Schouweiler. You only need the simplest of measuring tools– some drill bits. The following is paraphrased from several of Dave’s responses to block identification questions posed on the forum.
Remove the center freeze plug from the side of the engine block. Using common drill bits, then slip the shank portion of the largest possible bit in between the center cylinder cores through the freeze plug opening. The size of this largest drill bit indicates which water-jacket core was used to cast the block.
If you can only fit a 1/8- or 9/64- inch drill bit shank between the cylinders at the largest gap position on the block, and a 10/64-inch bit doesn’t fit anywhere, then they are 427 water jackets.
406/428/DIF361/DIF391 blocks allow a 13/64-inch drill bit shankto fit into the gap at the largest position.
MCC361FT/MCC391FT blocks (MCC = “mirror 105” marking) allow a 14/64-inch bit to fit between the cores.
Regular 360/390/410 blocks hang around the 17/64- to 19/64-inch water-jacket space at the largest position on the block.
These are only approximations, but tend to be close.
Even if you do have the good jackets, be sure to sonic map the cylinders before boring. If the core has shifted, it could cause problems. It is not at all unusual for FE engines to have considerable core shift. And the oft-raced and abused 427 engines seem to have some of the thinnest cylinders. A block with core shift has cylinders that are thicker on one side and thinner on the other. This can leave the cylinder wall too thin after machining, compromising strength and piston-ring seal.