Cadillac Repair: 1974 Eldorado overheating issues, professional machine shop, vacuum modulator

Question
Car overheats after only driving 8 to 10 miles at 60mph.  Engine just rebuilt by professional machine shop.  New water pump, fly wheel, alternator, radiator, fan, coolant, thermostat.  Thought issue was spring in old fan that was not working, but car still overheating.  Have a 180 thermostat in the car now.  Now tranny is not shifting car into 3d gear, so I think I may be running rpm's too high on interstate causing overheating.  Worried about heat damage to rebuilt engine, but oil pressure is good and motor sounds like it is okay except for gunked up carb.  Any other potential causes?  Mechanic says timing is set spot on.  Also, when car overheats, top won't go up and down, but top and motor are also new.  I am spending lots of dought to ride shotgun in roll back tow trucks....

Answer
I would replace the radiator with a NEW radiator NOTT a REBUILT one.

I would also have the carb cleaned or replaced as it could be running LEAN which will cause overheating.

I don't remember if your transmission has a vacuum modulator on it or not. If it does, The steel line that goes from the engine to the modulator developes tiny pin holes in the line and causes a loss of vacuum to the modulator. replace the line with some rubber hose from the engine to the modulator and replace the modulator also.