Well my first reaction is that your system is working properly. The early 124 cars make absolutely the worst conversions of any of the hundreds we have done.
If I forget about the obvious, I would go next to the recirculate door activation. It is a two stage vacuum actuator. It is usually somewhat closed at first stage. It goes fully closed at full recirculate. If the door is open the duct temp will be anywhere from 10-15 degrees higher than it should be.
Bosch Master, ASE Master, L1
33 years MB technician