Are you sure the guage is reading correctly?

If the engine has JUST been rebuilt (i.e. you are in the process of breaking it in) it may just be a little tight as everything wears in). Try some short trips when the weather cooler and get a few miles on it to break it in.

Is the temp at idle ot running down the highway?

What type of fan and shroud (if memory serves correctly that would have a fixed 4 blade and no shroud stock)

What are the weather conditions and driving conditions? (100 degree days at 75 MPH up hill will tax any cooling system especailly in an 4000 pound car).

A picture of your engine compartment/cooling system might be helpfull.