A number of things can cause the engine to not idle up including low fuel pressure, timing, vac leak, or sensors that are not connected/working properly...could also be your throttle cable is either too slack or there is something wrong with the way it connects...but that would be too obvious...
If nothing else has changed and it has not worked since you put it back together you might also want to unplug the battery for about 20 minutes to try to reset the computer...it might have set itself for the old head specs (air/fuel mixtures and O2 sensor levels) and simply needs to be adjusted to where the new head is at...