To get it to be letterbox, you can just drop the footage on to a 4:3 timeline and FCP will automatically letterbox it.
As for getting it to NTSC you have a few options. Nattress Standards Converter is a good one.
http://www.nattress.com/Products/standardsconversion/standardsconversion.ht...
You can also export to NTSC using Compressor. Another option is to use the Conform 25 to 24 tool found in the Tools menu and edit your project at 24fps NTSC.
Samuel Hall
DP/Editor/VFX/Music
www.therevision.net