My turn to rip my dash apart for this repair came last week, only I did not have evaporator issues. Instead, my heater core was leaking. I took no chances--I replaced both the heater core and the evaporator.
Thanks to "99GreenMachine", who sold me the foam gaskets, my heater box bolted right back up tight to the firewall.
A couple of tips for those contemplating this job:
1. have some spare 1/2 inch thick self-adhesive weatherstrip tape around. You will need this to seal the heater core and evaporator where it slides into the heater box.
2. have some 1/4 inch thick weatherstrip as well. You will need this to seal all joints where the blow-molded air duct sections slide together.
3. buy some self-adhesive felt sheet, and cut pieces to fit whereever two pieces of plastic join each other in the dash. This will avoid squeaking once assembled.
4. the driver's seat does not have to come out, nor does the steering column need to be totally removed. Just push the seat all the way back, and drop the column on to the seat.
It took me about 3 hours to get the heater box out of the vehicle, following the procedure in the FSM. I spent the next 3 hours cleaning the heater box to get rid of the dust and mold inside. Then I found out the heater core I bought on line did not have the correct bends in the copper pipes feeding into it! That stopped my reassembly for the night. I would up getting a core at NAPA the next day instead.
Final dash assembly and coolant flush and AC recharge took me about 4 hours. I went carefully making sure that all the plastic bits snapped together and/or screwed together with the right foam or felt insulators.
I think the AC is working OK, but it's a bit cold (even here in Phoenix) to tell if I am getting the correct pressures. The pressure chart in the FSM does not go below 70*F, so I had to guess.