You'll never get a stock 200 to rev to 6K. You could get a modified 200 to 6K if done right.
Do the usual high performance mods to the bottom end: balance all rotating parts, chamfer crank oil holes, polish crank, dual link cam chain, ARP rod/crank/head bolts.
Cylinder head will have to be higher flowing than stock: CI alloy head, Aussie 250 2V, or if your really bold the Aussie crossflow. Don't forget to run roller rockers for reduced friction.
To make it worth your while, get a cam that moves the power band higher into the rpm range like a 274/274 (2000-5800), or a 280/280 (2300-6000). Definitely have to run higher numerical gears (3.55-3.73+) in the rear end because it will be a slug off line without them.
Don't forget headers, low restriction exhaust, upgraded ignition, and upgraded carb(s). I would also suggest an electric fan; wouldn't rely on the stock fan staying put at 6K.
I'll also ask why you want to go to 6K?
My BMW X3 3.0L is rated for 260HP @ 6600, but I seldom take it up that high, 0 to 60 mph in 7.1 sec, 80MPH at 2800rpm. It does make a sweet sound though getting there.