This sorted out my TC cutting in - I had to change the tyres to the ones recommended below.
Stock 225/40R18 (25.1) 235/40R18 (25.4) .3 inches
18" opt 245/40R18 (25.7) 285/35R18 (25.9) .2 inches
19" opt 1 245/35R19 (25.8) 275/35R19 (26.6) .8 inches
19" opt 2 245/35R19 (25.8) 295/35R19 (27.1) 1.3 inches
20" opt 245/35R20 (26.8) 285/30R20 (26.7) .1 inch
First and foremost, you can go very, very wide on the car if you follow a few guidelines. First and most importantly there's the traction control issue. Putting the wrong size tires on the 350Z results in the traction control cutting in and disabling power to one or more cylinders. All but one model of the Nissan 350Z comes standard with traction control that is not easily disabled. The easiest way involves disconnecting several connectors, one of which includes the brake lights. We won't even go into how to do it since it is outright dangerous to drive a car with no brake lights! That being said, it is easy to get the tire size right front to rear, and here's how you do it. The traction control looks at the rate the front and rear tires are spinning and compare them to each other. The stock tires have slightly different overall diameters, and the computer knows this and calculates the rate with this in mind. The stock 225/40r18 has an overall diameter of 25.1 inches, the rear 235/40R18 has an overall diameter of 25.4 inches, so the stock tire is a little over a quarter inch larger than the rear. When changing to differently sized tires, you MUST keep the front and rear over-all diameter within 1/4" to 1/2", or the traction control light will cut on and the computer will cut power to the cylinders of the engine. Since the rear is not actually spinning the car will never stop cutting power, and the VQ35 will act like a 4 cylinder rather than a 6, if not worse.