One thing that always helps is to add manual cut-off valves to the heater hoses from/to the engine. The heater water valve under the dash does not have enough throw in its levers to completely shut off water flow. If adjusted to do this, the valve will not turn on completely when the heater is selected. An added safety factor is, the two short rubber heater hoses under the dash tend to deteriorate and are not visible, so they get skipped when maintenence is done. If one of these hoses bursts while driving, it will fill the cabin with steam, opaquing the windshield, and may scald the drivers lower leg. This is more apt to happen at high rpm. Injuries have happened and cars wrecked- enough said! A 2nd thing- if you have converted to R-134a freon, a new larger condenser is often needed since the eff. of the 'new' freon is lower than the old R-12 9no longer available)and it also leaks easier unless barrier-type hoses are used.
