You’re right to be concerned about moving the chimney too far back. At the same time, you should not eliminate the firebrick core, because that gets extremely hot. (Fireplace chimneys include a refractory liner for the same reason.)
We’ve looked at these bricks before, and I still think they’re gorgeous. You do have the option to make “splits” for the front aspect of the chimney. You’d get one face for each brick, but you’d be using a single brick there either way, so it’s not really a waste. Your option whether to do splits on the other three sides of the chimney core.
If you go this route, you should not have to move the chimney opening back at all, which I know @BrickWood would prefer.
A follow up question, Dave: do you plan to close off the oven front? If you do, you might be able to avoid making splits altogether.
Again, love those bricks!