The OEM blower motor was a two speed unit but the new replacement motors are three speed units. The most seamless way to handle the update is to use a Pantera Electronics Console Switch Controller. It is designed to work with the new three speed motors.
Also, the logic behind the aftermarket heater core / evaporator is, the evaporator is 50% larger than OEM and the heater core is 50% smaller, all in the interest of helping the A/C work better. I can't tell you how effective it is though. Perhaps someone that has one in their Pantera will chime in.