I port with rifflers mostly as there are not that many small diameter long shaft die grinding tips. For transfers you can open up the window side and then attack the port side through the port from below. Take your time and it is not so difficult, if you are in a hurry to get done some of the mistakes take longer to correct if they are indeed correctable.
Boring a stock bore to such a big oversize might be a bit of a hassle, you could run into some casting voids or deviate and have too thin of a wall as well as a untrue bore, not many shops have a boring bar that small so they would be cutting it open ended.