None...go with a straight through muffler like a dynomax ultraflow, borla xr-1, DNA mufflers, etc...sound way better and actually help horsepower instead of restrict it
Glass paks are straight through mufflers, the only difference between the traditional glass pak and the mufflers mentioned is the size of the case which provides for more packing and a quieter longer lasting muffler.
There are some real cheap glass paks that have a smaller iternal tube than the connectors that you would want to avoid but it you like the sound and don't mind that they get louder with age go for it.