Can I put in a factory A/C unit into a truck that did not come with it? I mean what kind of parts and modifications need to be made? I need an outside A/C Box, compressor, factory drier, condensor, what goes inside? Do I need to replace the interior box? Switches? any info would be helpful.

You need to change out side box, I also believe the inside too ( i am pretty sure of it).
You need to change heat control switches.

Also need to modify fire wall in heater box area.
and then a whole lot of parts like hoses, flappers and engine ac unit parts.

LMC truck has a good breakdown of the parts, (less the fire wall mod).

Best bet is to find a truck with ac and strip it out.

Good luck with it.

or go aftermarket............many choices