I used Corvette C5 belts in my GTM. They look "cleaner" than the race harnesses, and fit the look of my interior a little better. The thing to watch out for when using them, is that they MUST be mounted at specific angles. By this, I mean that the retractor has to tilt towards the rear and towards the outside simultaneously. I was fortunate enough to know a larts genius who had already thought through the mounting requirements for the GTM. I BOUGHT A SET OF HIS BRACKETS, and they work fine.
You would have to run a trial and error process until you find the compound angle that lets the retractor wind in and out freely. When you find that, the retractor ballancer will cause the roller to lock under heavy braking, preventing the webbing from moving, and holding you in place. If you're interested, I'll send some pictures later tonight.
Mike