Is there some reaosn to think it is not a 1977? If you can find the
merc tag with the serial number on the engine that will help you. If
you think the engine has been replaced at some point and there is no
tag then you may be able to find a number stamped in the block that
would give you an idea. It's usually really hard to find these except
with the engine out of the boat though. The 3.0 is a gm 4 banger and
is relatively unchanged over the years if you are looking for parts.

let wrote:
How do I determine the engine year?
It's a i/o with a 1977 pre alpha outdrive.
Engine is a 3.0L 140HP.
Thanks