The conversion for the -005 should be exactly the same as the -004; the schematics I used for the -004 cover both models.
This de-rating thing can be a complicated question. We have a few things to worry about:
1. The current through individual generator windings. I have not yet taken current readings and compared them at full load 3 phase vs single phase on each winding. I do intend to do this. I don't think this is an issue, as the zigzag configuration is considered "balanced" (although it looks a little strange on paper).
Edit: I have tested all 12 leads under full load, and the current was balanced to less than 1A between all leads
2. The cable size from the reconnection board to the breaker, the breaker itself, and the cable going to the load terminals. The breaker is not an issue. It can handle the extra current. The cable size is too small for the higher single phase current through L1 and L3. It should be up-sized. If you don't upsize it, I seriously doubt anything bad will happen...but you may experience a slight voltage drop across those lines at full load and/or some warming. I intend to put some 4 AWG cable on my -004.
3. The overload trip setpoint on the generator. I have not yet tried to raise the trip setpoints for the overloads (planned for this Sunday night). They still trip at their 3 phase setpoints. This is roughly 55A for the -004 (mine trip at about 60A), and 110A for the -005. At the new higher voltage, this gets you 14kW for the -004, and 27kW for the -005. This means that, at the new higher 120/240 voltage, the sets trip at just below their full rating. However...the set won't trip for a full 8 minutes of overload. That means that, on your -005, you can pull 27kW all day long and for a few minutes at a time put even more (40kW is possible on a good-running -005, 23kW on on -004).
The bottom line is, you can do the single phase mode I show in my video, move the leads on the reconnection board as I describe in post # 40, and put your generator to work single phase. It will work just fine this way, especially if you intend to load it to 50-80% of its capacity with only occasional 100% transients.
I'll have another video up 2 weeks from now describing/detailing the ENTIRE conversion, not just the zigzag mod shown in the first video. Should be fairly easy for the average wrench-turner to accomplish.
Edit: New video is at the end of post #1