The bolt pattern on the cranks are the same. What changed was the diameter of the centering register on the end of the crank. It got larger in '68. You may not need a different flywheel if you have a '68 up 170, and you won't need a new clutch unless the one you have is shot. If needed, a later flywheel would solve the problem and probably be cheaper than an adapter ring. Sorry, my bad.
Count the number of freeze plugs under the intake. '67 and older had 3. '68 up had 5.