Were the condenser & evaporator & hoses thoroughly flushed? The MB AC conversion tech sheet says to remove r12, pull vacuum, add captured r12 back in, and remove it a second time to extract the mineral oil in the system, but that is just not enough. The evap, condenser, and hoses should have been flushed separately a few times to get the crud and oil residue out.

Next, did you remove the wiper mechanism and blower motor to clean the inside of the heater box? Dirt has been building up on the fins of the evaporator for 25 years. Spray AC cleaner foam in and flush slowly with water. It drains out the two drain hoses.

Hopefully, you will return to "the best of times" from this "worst of times"!