Pretty neat, That looks good, Can't wait to see smoke coming out
I'd be careeful limiting the air flow too much. I can say that your intake should be okay to cut down. I'd be hesitant to limit the exhaust. Nothing worse than stale smoke. You have a large uninsulated area to heat so it's going to take longer ad more BTU's to get you up to where you want to be. I would fill in your conc. block with some sort of thermal mass. That will help you maintain temps once up to temp, but will also take longer to get to temp. Perilite or something similar might work best, as it would retain heat and would absorb heat faster than say sand or conc. If you find you need to insulate rigid foam against your existing walls with plywood over would work.
Even my big block GOSM can eat through a tank of propane when its cold, and its probably only half the size of your shed.