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I've never had a cooling problem with my latemodel 604. I put a new engine in with new Jones pulleys and 17" hustler fan and cant get it under 210*. I run a double pass rad (3 years old) with shroud. I'm told to go back to 4 blade fan but i see lots of people running the jones. Or swap out to a new rad. We also found the new serpentine belt was slipping so we were tightening that as well as trying different restrictors and no restrictor, nothing helped.
-what water pump?
(with a double pass rad, you probably need to ditch the restrictor. Stewart stage 2-3 water pump is best option, and they tell you no restrictor with double or triple pass.)
-are the pulley's a reduction or 1:1?
-the hustler fan does not have the pitch or the tip design of the GM 4 blade. A lot of the magic of the GM fan is in the tip of the blade as well as the pitch.
Go to Wal-Mart buy a kiddie pool, (or sneak kiddos pool in shop one night), and fill it with dawn and water. set radiator in pool completely submerged overnight.
next night wash out very well from the fan side, then blow it out with air hose or leaf blower.
3 year old rad, may have a lot of debris in fins restricting air or heat transfer.
Just a couple thoughts to try