Pulling the dash is the best option in my opinion. That way you can remove the hvac box and replace the coil, heater core, blend door motors and accompanying flaps, as well as the weatherstripping...you can probably access the evaporator coil by just rotating the dash down, but i have never done it that way... Getting the plenum box halves separated without breaking it is rather difficult even with it out of the cab. Plus replacing all the old crusty stuff in there is usually necessary after 25 years. Replacing the coil, putting dash back together and then having the blend door not operate would irk me quite a bit...I don't think pulling the dash is that hard, can do it in about an hour now. there are a few good threads detailing the intricacies involved on this site.
