Best I can find, the C6ME blocks can be about anything. I've got C6ME-A, which looks to be a .060 over 390 (or 410 - it's got the 1U crank). That number shows for everything from 390 through 428, and I've seen indications it's also used for the 352. The test I'd read was to see if you could fit a 1/4" (some sources say 13/64") drill bit between the cylinders through the freeze plug holes, but those can be hard to get to...

Anyway, a "C" on the back of the block (also hard to see while installed) reportedly means reinforced main web 428 (CJ), or 390 if the casting date is early. "A" scratched in the sand before the block was cast supposedly indicates a standard 428, and no mark could be a 390 or a 352 if the casting is early.

Good luck!