I think one of the folks on the board looked into a new heater box while replacing his evaporator. (Aldemon, if memory serves.) His dealer wanted $2200 for the assembly. Assuming the price is correct, I don't think it makes much sense. A good evap and new vacuum elements run about $400.
It's all back in and working great. No real difficulties putting it all back together. The heater box slides right in and bolts up to the firewall easily enough. From that point you just re-assemble the interior around the heater box. The toughest thing is probably to insure all the electrical connections are back together - there's a lot of loose wires hanging around!
I'm planning to replace the compressor, manifold, condenser, etc., but just for kicks I hooked up the vacuum pump to see if it would hold vacuum. Pulled it down to 29.5" and closed the valves. It dropped to 29" in the first hour, but has been holding there for 24 hours. It would never do that with the old evap.
It took about 18 hours of labor to do the job. So it's a big task, but not an especially difficult one; most of the work is pretty straightforward. The biggest challenge is just keeping track of the huge mass of parts and fasteners to insure everything goes back into its appointed spot.