Starter current is something I've wondered about, too. In my case the engine is a Cummins ISC 8.3L. I've seen the occasional bit of information calling out this starter as a 6.4 kW unit (there
for example). Supposing the battery drooped to 10 volts while cranking, it would be in the neighborhood of 640 amps if the starter really pulled that rated 6.4 kW. But I don't know whether that would be "normal" or whether it's merely the maximum limit of the motor and we'd normally crank at a much lower current. I suspect the latter. So is normal 300 amps? 500? I'd love to see some data.
Interesting points made about switching positive side vs negative side. Here's another to consider. Virtually everything conductive in the area of the battery is connected to the body ground. Suppose one disconnects the negative cable before beginning work. If a metal tool is dropped and it bridges from a battery terminal (either terminal!) to some surrounding metal, it's no big deal. Either the tool connects the body to the positive terminal, which is fine because the negative terminal is disconnected so no current flows, or the tool connects the body to the negative terminal, which is also fine because that's what will eventually happen anyway when the battery lead is put back on. Accidental shorting either
terminal to the body, engine parts, brackets, etc is safe. On the other hand if the positive were disconnected and the tool dropped, there's a 50/50 chance it'll be shorting the positive terminal to the body, which is wired to the negative terminal, and Bad Things may ensue.
A note about the original wiring diagram: it's slightly superior to take the positive lead off the post on one battery, say the one on the left as drawn, and take the negative lead off the post of the battery on the opposite end of the chain (ie the right side). This ensures the batteries will be equally loaded. When connected as in the diagram, that battery on the left will carry more of the starting burden than will the one on the right because of the extra resistance of the cables and clamps going to the battery on the right.