I just did mine. Full disassembly and pressure wash sand blast to get it clean. Cut the top off with a band saw and used a piece of 1/4" cold rolled flat bar to cover the hole. I did not go as low some others have gone as I wanted to keep more of the case. Preheat was somewhere in the 500 degree range, I used a propane torch inside the housing with a heat blanket covering it even while welding. Some people have used a gas BBQ and cooked it like a turkey or pig. Then TIG welded with 309 rod. I forgot to needle gun it afterwards. Put a fire blanket over it and let it cool covered till the next day. No cracks or pings developed.
^ Similar, but I used a thicker plate (what I had on hand) and Mig'd it with a good PH. Held up just fine behind a doubler and have over 20k miles on it when I sold the truck. Here's how I did it with mine: Edit to add: http://midwestnomads.com/2016/04/the...ing-the-np205/
Edit to add:
For the Pre Heat, we just used a Rosebud tip with a torch to get it good and arm (used an IR Gun to make sure it was warm). One guy welding and the other guy doing "touch up" re-heats to keep everything warm. It welded beautifully.
I was not as methodical as I probably should have been, but its been holding fine for 7 years with no issues. Preheated with a torch. Plated with 1/4" mild steel. MIG welded with .030 ER70S-6. Wrapped in a blanket to let it cool. I cut it about as much as I could trying to get it clocked close to flat. Hard to get a good picture under the rig but you can see how much was cut.