I'm runnin 33's ......not a problem yet. I also asked this question a million times, and usually was told to run 33's......but i guess it depends on how hard you beat on it off the road....and 33's are about as big as you can go without doing some lifting....and they will rub at full lock (no big deal)
This is probably coming from one of the only guys who hasn't busted a birfield yet......so i'll keep you posted. I drive my heap of termites about 15 miles to and fro work every day
and no problems yet.
Birfields really don't break as often as people think they do. If you are sticking with a F or 2F, lay off the throttle and don't reverse at full lock up a hill while laying down the throttle you probably won't break birfs. Supposedly front lockers will aggravate the situation in general, there are a few rare scenarios where open diffs are worse. Ignoring body limitations, the Cruiser drivetrain can handle 35's easily (might want lower tranny gear than the 3 spd/4spd offers,) Woody runs 39.5 with stock drivetrain other than transmission and engine I believe. You really don't need to worry as long as proper maintenence is done and things are in good shape with any size tire you would want to run on a 40.