I've only had mid-dark blue and black cobra bodies...no problems with QLD heat in my experience.
Black seats, well if you are leaving the car out in the sun, get some nice labelled/logo'd towels to throw over them or get a torneau cover.
By the sounds of it you guys are building some very nice Cobras, can't wait to see the finished product.
Mine Cobra is progressing, I spent a good few hours on it today...life is good. Can't wait to get the Halibrands & big rubber onto it.