Hi geo,
According to my Jerry Howell V twin plans the centre line is 1.680"
I can see your logic behind the figure of 1.670" because if you divide the overall height by 2 this number comes up.
Don't be fooled, when "the centre line" statement is used this does not always mean that it has to be in the centre of the part.
Although I personally have not made a start on this engine yet it has been made successfully plenty of times.
Jerry Howell was an extremely talented man, I am sure if you follow the plans exactly as they are there should be no issue.
Good luck and if you are underway with the build please post the progress.
Cheers,
Baz.