Hi mate,
Firstly i used to have the same issue in my old gtr,
Turned out to be the radiator's top tank was clog up, i removed it, took it to a radiator shop, had the top tank removed, clean + new seal, $85 and the heating issue was gone.
Also after you've flush the radiator, while toping up the coolant again, did you jack the car up a bit, had the heater at full blast and bleed it through that little screw on the side of the block?