You want to check the cyl wall cores through the blocks freeze plugs. C6ME was a generic casting number used on many differnet bore blocks. If a 13/64" drill bit shank fits snug between the cyl wall cores the block has 428 wall cores. If a 17/64" drill bit shank fits snug it has 390 wall cores. This is known as the drill bit test and short of a sonic map a very easy way to check what wall cores were used. You can resize the 64'ths to suit your bits. It's just the standard measurement used. 6A22 is a 66/Jan22nd date code. Value would depend on which block it is. 428's are $500 and up. 390's are $100-200 for block only.

G.