X400 on the new core. Also while you are at it, think about replacing the blower motor, since it's a PITA and requires the dash to be about as apart as the heather core does.
New core and blower and presto - hot enough to survive a real winter if need be.
Also, yeah - A/C works better for defrosting because it turns on the evaporator/dries out the air. Put on recirculate, turn on the A/C. Set temp to *hot* - you'll get warm and dried air.