I have one of their mild steel 4 inch shafts. Wanted to buy the 3.5 inch chrome molly and they told me I was too close to the critical speed to run a 3.5 inch shaft. My shaft is pretty long and I probably would have had the same issues as Al did.......

67 Coronet 500 9.610 @ 139.20 mph
67 Coronet 500 (street car) 14.82 @ 94 mph
69 GTX (clone) - build in progress......