I just ordered the D450's for my 66 from Summit and they honored the "clubgp" discounted price. You have to walk them through the process to add the code.
Tell them to add the item to the shopping cart. It then brings you to the order summary page. There is a button for "Recalc/Update Order". Have them press then enter "clubgp" in the discount code box and press the submit button and the discount code will show.
FYI - the D450 ceramic coated headers are $754.36.