http://www.offroaders.com/tech/12-volt-wire-gauge-amps.htm seems to indicate that 200A can go through a 6ga wire up to 10'.
Except the load isn't JUST the + line, it's the COMPLETE circuit... back to the ground or negative... So you're looking closer to 15 or 20', and that doesn't take into account what the ground wire is that you have from the battery - to the block... as the block is used as the ground for the plugs. IIRC, the wire that goes to the frame is something like 10ga.... It would be a good time to do the 'big 3' upgrade....
In addition, those charts look like they're using a 10% drop as acceptable, which it isn't, no more than 2% drop is suggested, IOTW your 12V will drop 10% down to 10.8V...
Besides that, 6ga is WHOLLY INSUFFICIENT for 200A, period.
Have someone hold that wire, I'd bet any amount of money it gets hot when you run the glowplugs..
Resistance creates heat, that heat then further raises resistance.
I would hesitate running 200A through 4ga, 6ga is not an option, in my book.