I have not done this following 83-100. But I have had 2 evaporator boxes out of 2 W123's.
After looking at my pictures I took over the course of the 2 removals,
make sure that both side bolts have been removed and the 2 brackets are free to separate.
The defrost duct must move downward to get it to separate from the dash. It does recess into the dash when installed. Or push up on the dash in the defroster area.
The heater box and evap box are held together by the 2 brackets shown in this link picture 110 and 111

http://www.peachparts.com/Wikka/W123Evaporator



The nut removed is around a stud that the bracket must come off of before the unit can move. Make sure the brackets can separate.
Also make sure the 3 hoses have been disconnected and they are free to move back into the cabin.

Dave

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1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car