let me throw one thing out-- could the fans possibly be interfering with airflow at highway speeds?
The way this rad is constructed, the three fans are mounted on the face of the rad--- the shroud is a metal covering the areas between the fans, with flaps on rubber grommets that open up at speed.
The fans turn on at 190 degrees. In my old application, without the metal shroud, this wasn't an issue, becuase other than the fans mounted very close to the rad, there was a ton of airflow. With this new rad, if I'm going at highway speeds, and the fans kick on, won't the fans interfere with airflow? in other words, if the air speed coming in at highway speeds is 55 mph, and the fans are only capable of drawing air in at (whatever) 20 mph, I'm thinking this may interfere with airflow at high speeds-- the only flow through the rad at highway speed when the fans are on would be through the little flaps.
I'm debating putting a relay and a switch on the fans and try turning the fans off on the highway, see if that does anything... but before I bother,m would love to get input.
By the way, this is the rad I'm using -- 2 row aluminum novak-- specifically designed for a 350 conversion in the xj:
http://www.novak-adapt.com/catalog/cooling/radiators/xj/k-xj31h.htm
this is the shroud (listed near the bottom, fx3400):
http://www.novak-adapt.com/catalog/cooling/radiators/xj/k-xj31h.htm
Novack installed the fans and shroud and shipped as a unit, this is it installed: