Since no one has replied to your question I'll throw in my two cents. Hardened valve seats aren't necessary. As far as the rest of the thing you want to do, all are beneficial but only if you replace the cam to take avantage of the upgrades. You can keep the stock bore and you already have the increased stroke with the 255 Merc engine. You might check into the Isky Max-1 cam shaft for a good streetable engine.