I've only assisted a buddy on a 126 evaporator replacement (~17.9 hrs. book time), but I know the 140 is markedly tougher to do (I'm certainly no expert, but I hang around people much smarter than I am
). Entire dash disassembly to the "core" (a little MVAC humor, very little) is required. It is verrry difficult. You can't really get to it without disassembly, so it is usually checked with dye at the drain tube. As I mentioned before, make doubly sure it is evaporator before losing sleep. ( Although, unfortunately, it sounds like it is.) As far as location, evap. is dead center of cabin side of firewall, inboard a bit. If you decide on the "FULL MONTY" have dedicated Indie or trusted dealer do it. If you try the sealant route (many 140 owners have, rather than rebuild) try an A/C shop with a good attitude and lots of experience.
By the way, I believe it's 25+ hrs., not 20 (I could be wrong)