Clearance won't be any more or less than stock, with the Cal Trac set-up, even if you use their springs. I have 15"x8" rims with 4" backspacing using 275-65/15 MT ET Streets on a 2" narrowed rear (1" each side). I can get my thumb between the tire and springs, so they're safe.
There are lots of cars with lots of street miles on them, using Cal Trac springs, and I've never personally heard of a failure. I really like mine, but admit I don't take 1000 mile trips in my car. Jay has plenty of Drag Week miles on his rides and hasn't had an issue. Either way you go, using just the bars, or the bars and springs, I don't think you can go wrong. But I will say that if you want a soft factory ride, you may want to stick with the factory type springs and just use the bars. The Cal Tracs do ride a bit stiffer, but they're certainly not overly harsh. My old springs, with an extra leaf added, rode much harsher than my split mono-leafs.