A few things that will cause vibrations are: bent shaft, bent or out of balance prop, 2 blade prop, worn strut bushings, bent or out of balance drive shaft, worn universal joints. If you're running a blown BBC DON'T run a bronze prop, the blades are likely to break off and cause much damage. You need a steel prop if you are running faster than 65/70 mph. Also don't run a safety collar in front of the strut, it will keep water from getting into the strut bushing and wear it out prematurely. Check your strut bushing for wear. This should be enough to get you started.