A slight port actually allows the horse's tongue to be relieved of pressure when the bit is pulled. A port that is smooth and slanted slightly backward is more comfortable than a port that is straight up. But a port allows room for the tongue if it is fairly wide and comfortable for your horse. The bit then puts pressure on the bars, not the tongue. You need to find which mouthpiece is comfortable for your horse.
High ports are likely to touch the roof of the mouth.
A curb bit should have a moderately thick mouthpiece. You need a thickness that suits your horse. Too thick a bit really can overload a horse's mouth. Bits made of sweet iron, copper or stainless steel are most comfortable. According to Harris, aluminum tastes bad to horses and dries the mouth.