You can do it, just takes some time. I didn't take the entire dash off but you can. I just took the 5 or six screws out on top of the dash, drop the steering column, and the two bolts on either side of the dash in the kick panel. Then rotate the dash back as much as possible. Then we disconnected the heater hose and cut the tubing in the truck with a saws-all and unbolted the core and pulled it out. If you get a replacement from Napa they have swivel hoses on the core which make it hell of a lot easier to put back in. It will be frustrating to get the new one it but it can be done. So far I have done a 95 cummins and my 02 cummins.
Here is a video on it, not sure how much help it will be my internet connection is to slow to watch it.
I also did mine in a day. I didn't replace it, but I had it out. My evaporator was plugged solid, and I had no air flow. It's leaking a little now, just enough I get a wiff of coolant when I switch to heat.