I think it needs a 6-3/4" pully. Sounds like yours pump is cavitating from spinnig too fast. The 216 fan is not designed to spin that fast either. I used a short pump from chevy duty, but I still had to move the radiator forward. If the larger pulley doesn't work, I would try the adapter kit for the stock 216 pump. Seen em on ebay for $40.
