Every Brinkman I have seen is known for their leakage at all doors, ports.
I bought a "Brinkman SmokIn-Pit Pro" many many years ago before I knew better, Later I did a couple mods that I found on-line for heat baffles and door seals and I have made it into my seafood smoker because of it's long length.
Can't manage a long burn at higher temperatures with it, but it works well for cold and short low temperature smoking using charcoal.
I wish you luck, let us know how remounting the firebox works for you. Keep us posted on your progress..
High temperature RTV sealant works fine and I used NOMEX gaskets around the doors.
Humphrey's DownEast Beast W/BBQ Guru