I do not understand why you would want to drop to a 38 when you have a 36 middle. I understand you are not pushing big gear inches, but the 38 makes the gearing awfully low. I also am not familiar with 104/64 front, ergo, I do not know what rings might be available. Maybe look for a 44 tooth big ring. Sometimes you just have to play around to find what is going to work best for you.