Rf will provide more even temps...many people who build a traditional offset spend a ton of time after the build making tuning plates so why not just do it right feom the begining?
Firebox size is fine...the top of the baffel plate is typically straight i line with top of firebox. Shape of throat is of no matter.
Gap for baffle plate should be same size as throat...no bigger.
I recommend top mounted exaust...dont want to trap the smoke.
Let us know how we can help.