I have an APC 18x6w on mine and the SV-26 spins it at 7400RPM. I would check your prop and make sure it is balanced correctly. A prop that is not balanced with vibrate badly.
I am almost finished with my Stinson and I can wait to going flying. The plane weigh's 12.5 lbs. I is very cold here now and I don't like flying in cold weather so it my be a while before she goes up.
I'll post a pic when I have finished the build.