If you mean the smoke box has a hinged door, you could seal that up.
I would not weld up the firebox door, thats how I empty the ashes.
I would try blocking off the upper part of the firebox opening with something temporary like a cut foil pan to see if that helps. it might choke off your air flow
I think that opening is way too high, I dont know how well it works like that
Of course if you could cut the firebox off and lower it that would be good, but i would experiement with blocking part of it off first