It sounds like you have three different gaskets. I had a problem with my carb and the guy who built my motor gave me one that sounds like the 3/8" one you mention. I've gotten other gaskets for the same aplication that are different. The thicker one may be for a certain year stock gasket but the racers use them because it is stock and you get better performance by raising the carb. I would recomend you use one gasket, the thick one. I'm a rookie so if I'm wrong someone jump in. Pete