Why would you want to use a Brinkmann water pan in a large WSM? (Does Brinkmann even make a pan that fits a big WSM)?
The stock Weber 22 1/2" WSM water pan is very good. It has a large capacity and does not block access to the charcoal ring.
The stock 2009, 18 1/2" WSM water pan blocks access to the charcoal ring. Many people (including myself) use a Brinkmann charcoal pan, in it's place.
Bob