Definitely put the flat pan at the front over the fire box. I just built a homade arch to fit a 2x3 flat pan and it boils anywhere from 8 to 11 imperial gallons per hour depending on the auf and aof setup and how much I pay attention to firing. I have a pre heater tube around the stack which goes into a half size steam pan between the flat pan and the stack then flows into the flat pan at around 100* f.
Works great
2019 - Barrel evaporator 2 steam pans 44 taps 13 Liters syrup
2020 - Barrel evaporator 2 steam pans 51 taps 21 Liters syrup
2021- New homemade 2x3 evaporator and flat pan 80 drop tubes to buckets
2022- (•,•)1350L naturally ROd sap 44L syrup
2023- "\_(°•°)_/" 1100L sap 30L syrup not accurate due to natural RO
2024 { ';' }