I just had this air conditioner conversion kit professionally installed in my 1986 mustang GT. It seems to be working okay, but the true test is coming in July and August in Texas, right? :)
I notice that the airflow varies wildly and seems to be quite sporadic, especially on the "max" setting. Is this normal? It will be blowing full bast and then dip down to almost nothing and then back up to somewhere in the middle, then full blast again, etc. I don't remember the OEM A/C doing anything like that before the conversion.
If the volume of air coming out of the vents is what is changing, that will not have anything to do with this kit. Either the blower motor is acting up, or you have a vacuum leak that is causing it to revert to defrost under moderate throttle. If the airflow varies based on how much throttle you are giving it, it will be time to check for vacuum leaks.