do stock mercruiser 454s have iconel valves? i might throw a set of big ovals on my 454 but do i need to have iconel valves installed? or will they be fine in my mild application?

If your making any improvements over stock I would recommend them. Their cheap insurance. If you drop an exhaust valve it's going to cost you a lot more than a set of them to get running again. Marine engines make extreme exhaust temps.