my heater blows out cold air, why?

Is the engine coming up to temperature?
Were you having this problem before the rebuild?
Are the pipes to the radiator and from the back of the engine to the heater matrix getting hot?
It could be the heater matrix is blocked or the cable from the heater temp adjustment dial has come off.
Was the rebuild because of head gasket failure, if so did you use k-seal, radweld or any other 'instant repair'? This stuff can choke the heater.

freddie
May 2009