I've flown my regular O-360-powered Ranger without any heat problems at all, other than making myself too hot. My lowest departure so far was 8ºF, but I didn't think to check the OAT after climbout. I've also gone into Arnold Palmer in winter, arriving after dark for supper, only to find the ramp freshly plowed and the snow 8' or more deep. Even my wife was warm, and I think we took our coats off enroute due to temperature. Often we use about halfway on Cabin Heat, with the fresh air vent cracked to cool it off some.
So I hate to say it, but methinks you have a problem. Did you make sure all outside vents were closed? the scoop, the footwell vents, the cabin air knob, and the big lever under the throttle quadrant? Are your heat vents inboard open? There are valves there that if closed will direct heat to the windshield for defrost, one on each side of the center. Mine are black, nice and easy to see under the panel. :-)