In a message dated 6/2/2002 2:52:29 PM Pacific Daylight Time, ian.crowe@sympatico.ca writes:

<<<...I am aware of a limitation on the Piper Arrow with an IO-360-C1C from 2100 to
2350 rpm.  This engine is basically similar to my engine but drives a 74"
propellor...>>>

Unlike the -A3B6D, the -C1C has NO torsional-vibration-absorbing pendulous counterweights on the crankshaft, hence the critical excitation range (to be avoided) on the tach. BUT, be aware that the 68" prop will have a significantly lower mass moment of inertia than the 74" prop of which you speak, which has the effect of reducing the torsional resonant frequency of the engine-prop system (hence the tach range to be avoided). Also, without those "counterweights", the fatigue excitation applied to the prop blades is greater, and the blade resonant frequencies are lower than the 74" prop. Lots of guessing involved if you're not using a certified engine-prop COMBINATION.
Jack Kane