The one you're looking for is on the Options Tab. Follow the tree down: AVS Options/AVS Expert Options:Convert from LB 4:3 to 16:9.
You can also adjust for ITU aspect ratio and use any additional AviSynth filter you'd like (in case you want to tweak the picture while it's at it.)
I'd consider DVD Rebuilder a near on perfect tool if it would allow PAL -> NTSC (and back) conversion.
Check out these guide:
http://www.afterdawn.com/guides/archive/dvd_rebuilder_tutorial.cfm
http://www.afterdawn.com/guides/archive/dvd_rebuilder_tutorial_advanced.cfm
http://www.doom9.org/mpg/dvdrb.htm
They're not all up to date (settings have been moved around), but it'll get you started.