Re: Does coolant flow thru heater always?
Having also driven my 1970 in Hawaii for many years, I can say having the heater connected will not help with cooling the engine. It is correct on the A/C equipped cars, one heater hose had a vacuum-controlled shutoff valve that would stop coolant flow to the heater core unless the control slider was moved to provide heat. During warm periods, no coolant would move through the heater.
At what water temp will the electric fans turn on? Are the fans the puller type between the radiator and engine? How many CFM provided by the fans? Too little airflow through the radiator will cause overheating.
Retarded ignition timing and the use of ported, not manifold, vacuum to the ignition advance unit can produce extra heat.
Long tube headers radiate a lot of heat in the engine compartment. Perhaps consider coated headers or a header wrap to reduce underhood temps.
1970 Chevelle Malibu purchased new in 1970
355 SBC, XE262H cam, Performer RPM heads, MSD HEI, TH200-4R trans, 3.55:1 Posi, digital gauges, QA1 coilovers