The B designates a change from the original casting/design. There may or may not have been a
C8DE 6080
or
C8DE 6080 A
released to the public. Are you sure that number is 6090? In '68 there were only 200's and 170's. I know that somebody on here knows which one it came off of, but for the most part it doesn't matter, just have it checked for cracks and check the combustion chamber (CC) size to determine if you can use it. If the CC is too large you can machine it. If the CC is too small depending on how small you can either use a thicker head gasket or not use it at all.
have fun,
-ron