I have a non-turbo 740 which had similar problems. Turns out mine was all corrosion related - I started measuring block to battery resistance through ground, and had well into the 15 ohm areas. intake manifold to battery was closer to 400 ohms. I took a bit of 10 gauge wire, and just went and tied everything together, block, alternator, battery, frame, manifold, headlight grounds, etc.
Next start, I have GREAT lighting again, but the engine runs SUPER cruddy. So I disconnected the battery, and turn everything on that I could find - so as to discharge any remaining capacitance. Reconnect battery, fire her up, and it's running OK, but not great. Took it down the road, by the time 5 miles was past, the rig runs great again!
Read this thread, and while it looks like you've been dealing with corroded insulation, don't for get to take a look at the grounding straps and the reading from them to ground on bat.
Just a thought. works for me!