no heat in my 72 dart...AC car

Yep, the heater core is located inside the A/C-heater "box" that you suspected comes out. Having the A/C disconnected or completely removed has nothing to do with the heater working properly - totally separate systems.

You'll find two heater hoses connected to the core's copper tubes which penetrate the firewall. One of them has the water control valve mounted inline. I'm not sure on your '72 if the valve is cable or vacuum operated. Regardless, if the valve isn't operating correctly, it won't allow the heated water to circulate through the heater core. Take a look here and you'll see what I'm talking about: WATER CONTROL VALVE

You need to locate the valve and determine if it opens/closes properly. Do you know if the heater controls work in the dash? You said the fan blows, but does moving the temperature lever cause the blending door inside the "box" to move? Basically, the lever operates a cable attached to a door that pivots to direct hot air to blend with cold air to adjust the temperature.

Moving the control switch to "HEAT" should open the water valve and hot water flows through the heater core. If the heater control switch in your dash is vacuum operated, it's possible that one or more of the vacuum lines are disconnected or leaking. The old switches are also very prone to having their small plastic nipples broken off where the vacuum lines attach.

Do a quick check on these items and let us know what you find.