I have 1993 nissan skyline r33 non turbo 2litre?

heater problems, takes ages to heat up and gauge not accurate doesnt heat for ages and when gauge reads normal its not hot as themostat inside car says eg 32deg when a/c is on the gauge will go up to 3/4 to the H marks but engine not hot dont know whats up is it the themostat or sensor got anti freeze had the water flushed as i put another motor in and the heater hasnt gone right since then maybe themostat too cold or sensor not correct one can some1 help