vibration

I'm not familiar with the rubber spool mounts but, have you tried using a torque strap or even a turn buckle to temporarily lock the motor mounts just to see if possibly they are creating a harmonic through the chassis? It would be an easy thing to do and would eliminate the mounts.
I believe the spool mounts were a 73 and later piece? I have thought about your idea and its on the list of options.