I did that to my one-ton using a Hot Rod Air unit. They are now out of business. It has it's drawbacks but cools effectively. Its problem is the defrost. All the aftermarket units I've looked at use a combined heater core/evaporator to save space. The result on mine is that in defrost mode it doesn't drain water from the evaporator fast enough and you have to constantly fuss with the temperature knob to keep the windows from fogging up. And if using the cooling function with the heater to keep the windows clear, you can't set the temperature high enough to be comfortable without the windows fogging up. Works OK if it's really cold and you just set the temperature to max, which locks out the compressor, and crack a window.
These problems may be unique to my model but I've heard similar complaints from others with other brand units, most recently on an old Peterbilt with aftermarket air conditioning. "Defrost sucks", said the owner.
