Dan,
The turbo covers will give you very little added cooling capacity. People upgrade to these because they are less inclined to leak. The fins are designed to make the covers ridgid, not really to dissipate heat.
FYI ordinary '74s did not have an external cooler. The heat problems started in '75 with the introduction of thermal reactors. Later cars produce more heat because of the bigger, higher output engines and need external coolers.
Your temperatures are just about ideal. You have no problems, be thankful. Your car can sefely run up to 250 f for short periods. If you are not breaking 220, relax.
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Chuck Moreland '86 Cab - "Sparky", '77 Targa - "Sweet Pea"
[This message has been edited by Clark Griswald (edited 08-13-2001).]