It can be done but your gonna pay a smith to cut on a new custom action?? , If the action is new , I'd call Stiller and ask if he can exchange it for the Tac338 that is made just for the 338 Lapua , their a reason the Tac 338 cost a hundred dollars more than the Tac 300 cause it built up to handed the bigger round.
As for the bottom metal I'm pretty sure that HS makes a mag box and bottom metal that will feed the 338 Lapua