Did you generate the order or their sales staff.
If you generated it and ordered the wrong one, it's on you to return it. If their staff generated the order and you told them the engine that you had, it's on them.
If the latter, send them an e-mail back and state nicely that the sales person was told you had a 360/magnum whatever and they sent you a 318-340 pan, instead of the correct 360 pan. You don't feel that it's your responsibility to incur the return shipping cost in this situation and if they could please send you a return call tag.