GM did a 1985 Cutlass 442 concept car back in the day that ran 16x9 inch wheels, called the FE3X (Google FE3X and behold the most bad *ss looking G body ever made!). It had very minimal flares on it. I have 16x8's on mine with a 4in backspacing and can clear 245 50 16's without flares, but with minor clearancing of the inner fenders at the top, and rolled lips for about 8 inches (4 from the center on each side). I also pulled the lips outwards another 1/4 in, then re-made the contours using Bondo. No one ever notices it, and I can even run the stock stainless trims on it, with a similar cut to the lip of course!. You also would have issues of rubbing at full lock, but this can be solved by using a IROC-Z steering box, or changing the steering stops in the suspension. The Camaro box has it's drawbacks as it gives it the turning circle of a city bus, and you must run a G body pitman arm. If you want stupid-wide rear tires, consider mini-tubbing it. This would allow you to run over 300mm of section width, and still look respectable. Also, stay under 19 inches of rim diameter, as these cars generally don't look right with anything larger, unless you are going to do major surgery. Nothing looks worse than a car lifted up just to fit big, heavy wheels.
My engine is a mild 350hp SBC. I know this because it is nearly identical to one on Comp Cam's website which they used as a demo of the cam I have, and they had the dyno chart.
Oh, and skip the targa top. A G body may have a full frame, but it is not really strong enough to handle the loss of body rigidity that would cause. Plus, even GM could not build a t top car that didn't leak. In all likelihood, your custom job will suffer the same fate.