Good looking job, went heavy with the fire brick, that can't hurt.
Questions:
1 Are you using a special high temperature mortar?
2 Traditionally there is a vent for the combustion products I was wondering where this will be but I am sure you have that planned out?
3 Also you live in a frost area, will you be using glazed exterior bricks?
My uncle built an outside fieldstone fire pit oven and it did not weather well, the frost in the porous rock and mortar broke it up in a rather short period of time. Just curious, I love the project so far and am waiting for update pictures.