My grill came assembled as part of the deal. Luckily I don't have any leaks between the cook chamber and firebox. Just issues with the doors where I used high temp auto silicone.
Had a leak around the smoke stack too and just ran a bead around the outside and pushed it smooth with my finger. No more leak.
Unsure on how it would work (might have been this thread I saw it suggested on), but I read someone suggesting using muffler sealer. Not sure if it would give off anything noxious (sp?) stuff, but I know it is designed to seal exhaust tubing together so it does not leak. Might be a good cure for the leaks between your firebox and cook chamber. Maybe remove, run a bead towards the outside of the mounting flange and then bolt back together. Alternating between bolts snugging up everything a bit at a go and then tightening down good on the final run through. Sort of like cylinder head bolt procedure. Might be overkill, but at least you know you've done your best to make sure all is square and tightened true.
Best of luck with it.
Just did the first cook on mine yesterday. Spares and a brisket. Came out okay, but still not happy with my exhaust, tuning plates need some more work as well.