I am running the same configuration as you but with the standard viscous fan and no engine modifications my heater starts to kick in after about 4 or 5 miles. It’s no good idling the engine (even when up to temperature) as the heat is robbed by the fan the temperature gauge actually drops significantly when stationary. When up to full operating temperature the heater works well – in fact the foot wells on my 90 CSW get very hot and the trim etc is certainly warm to the touch. It’s fair to say that at low speed the engine does not produce enough heat in cold weather such as this to heat the whole car effectively but the front is warm enough.
Incidentally I would check the heater vent pipes going to the windscreen vents – they are known to become dislodged and leak reducing the effectiveness of the heater – all that hot air just circulates around the bulkhead and I have partially obscured the passenger windscreen vent to improve airflow to the driver’s side.