Help wanted with broken ac

All these cars have a low pressure cutout switch. When the compressor gets power it is in series with that switch. Evidently you have "just enough" refrigerant in the system to bring on the compressor, and when it starts to pump, it pulls down pressure in the low side of the system, below the switch cutoff.

You need to get yourself a service manual. And you probably need to pull the charge out and start over. WHY? Because you don't know what's in there. R12 has been outlawed now for many years. Someone else could have put most anything in there at this point.

Also look around "for stickers" or special fittings to try and determine if it's been converted to R134 or whether it likely still has R12 in the system

As old as this is, unless it's had hoses/ seals/ gaskets etc in the last 20 years, it most likely is leaky and won't hold a charge for long without further service work

You can download a 73 manual at MyMopar, but there is no 74. The 73 book is "close" on most of what you need EXCEPT WIRING. Wiring in 74 is very very different.