What is the timing at idle rpm(both with/without the vacuum can hooked up)and how high at what rpm?
Possible water flowing to fast at higher rpm's. Use a thermostat?
What is the rear end gear ratio? Do you have a tachometer? What is the engine rpm at 65 mph?
Clutch fan engages/disengages depending on air temp. Should be fine since it's working at idle. Its normal to disengage at highway speeds. Ok to have a ported over flow jug. 50/50 coolant and water is best overall for highest boiling point protection.
Only thing that stands out is the intake. The PPM intake is to big for that carb/cam/cylinder head combo. It's made to flow more air-n-fuel like a RAIV engine(230/240 cam/800-850 carb/big exhaust) would need. Can be the carb is not flowing enough fuel at higher rpms along with the huge intake plenum and runners. Try a stock or Edelbrock Performer intake.
