damn, all this work and you can't go any further
?
To get the box out you have to disconnect the a/c lines at the expansion valve near the brake booster and the heater hoses on the engine side of the firewall. How do you know that you have done all this work and not disturbed the a/c lines and caused the freon to leak out?
A couple thoughts, if you have gone this far you may as well replace the evaporator core, modify the center vent where it never closes automatically (but you can close it manually) and modify the recirc door where it stays in a position where it let's only a minimum amount of outside air in at any time. I did this on my 95 E300 and it worked out great.