Ford sells the evaporator, only already mounted inside the plastic plenum. At least this is so for the 1999. Huge job, I would think. A creative person may can come up with a short-cut.
Before doing this task, I would isolate the evaporator from the rest of the system and make sure it will not hold a vacuum ... proving it has the leak ... or by testing with leak detector. A big job to later discover you have replaced wrong part.