With my old motor, before I blew it up, on streets, in traffic it was fine, as soon as I would get on the freeway at or above 2700 rpm +/-the motor would start to heat up and above about 3300 the fan, a taurus fan, couldnt keep it cool on high. I cant imagine that I didnt have enough coooling capacity, in addition to the Taurus fan I have a griffin 1.25"x2 (i might have the wrong measurement here) radiator (supposedly equal to about a 5 core radiator).
I am planning on putting a new 406 or 418 together this winter and don't want the overheating issue to harm the new motor, as I am certain it was part of the reason the old motor blew. I do have an A/C condensor in front of the radiator so I'm sure that limits the capacity some. Anyone have any other ideas as to what may have been causing the problem

that is very odd, the problem is not your fan because at high speeds you should'nt even need it. what was the condition of the pump, hoses, radiator and the thermostate. if they are fine it could have been air trap in the cooling system.

Usually this symptom is indicative of an overly lean condition. I would first suspect the carb jetting was a tad too lean...especially the more rpms you turn. Could have been a vac leak as well.

I'm with Steve on this one, A/F ratio maybe even timing ?

thanks for the responses. Those issues should be taken care of with a new engine and some tuning.