Well, I am one, but not in a relevant field
It looks like you might be suffering from the dreaded Governor Droop. No, that's not what happens to old bodybuilders, and it has nothing to do with keeping your shirt on.
The voltage is dropping (probably) because your FuKing governor isn't doing its FuKing job (Did you say your Listeroid was a FuKing brand? I might be wrong there.) With that kind of a load on it, can you move the fuel rack and get the engine to speed up at all? If the engine speeds up, you still have some horses in there. Also, note the point where the exhaust starts emitting visible smoke. When you get to the point where you start seeing smoke, that's the practical limit for your engine.
George at Utterpower.com has an inexpensive governor modification kit that should cure your droop. It's only $10 at
http://www.utterpower.com/governor_kit.htmIn fact, I just ordered one today (Geez, I'm sounding like one of Willem's alter-egos!)
Some other things to consider are the type of belt you're using. If you are using a V-belt, expect at least a few hundred watts of loss there. A flat serpentine belt would be a lot more efficient. You said you were having a pulley machined, so maybe it is for a serpentine belt.
You say your gen head is only 2 years old. Does it look like the one in the picture I posted above?Â
Yes, when you load these engines up, they really don't complain. You can hardly tell how much they're pulling. Now compare THAT with a 3600 rpm genset!
Quinn