if you think about it its rather difficult for the adapter plate bolts to loosen as the header clamps them in place,
http://garage.grumpysperformance.com/index.php?threads/calculating-header-design.185/you may want to do the calculations on the ideal header design for your engines power range, it may not require a much larger diameter header primary diameter, but it might benefit from longer primary tube length and the merge collector with a longer collector.
doing the math before spending the cash is generally the smart route