Have you measured the bore size of the block?
From an old post by Dave Shoe:
C6ME-A was cast onto FE blocks for many years. If it's a C6ME-A block with a casting date code which corresponds to the 1966 model year, then it's logically from a 1966 vehicle.
Since any 1966 (or later) FE engine would have gotten the C6ME or C6ME-A marking, it could be anything from a 330FT, 352, 360/390/410, 361FT/391FT, 428, 427, or other. The sand cores stuffed into the mold determine what it ended up as.
Since your C6ME-A block has been identified by other means as a genuine 428 block (probably because it has a marking on the rear, such as an "A", "C", "X", then you're off to a good start. I can suggest other tests which would identify what it was originally cast to be.
Since 428PI blocks from 1966 were the only solid lifter 428 blocks found in cars, this would be a good verification that it's a PI. By 1967, the PI got the 390GT hydraulic camshaft and valvetrain, and this is seems like what would have gone into Shelby GT-500s.
As long as you are aware that basically all FEs in 1966 got the C6ME or C6ME-A marking, then you are on the right track. By 1967, C6ME and C6ME-A were still very common, but the C7 series numbers were also found. Same with C8. By 1969, Ford started to cast blocks without any CxME numbers, probably because the irrelevant numbers caused more confusion than anything else.
JMO,
Shoe.