Firstly standard def video on tape is 720 x 480 (NTSC) and 720 x 576 (PAL). Get your calculator out and you will find that is a 4:3 ratio. To get a 16:9 image to tape you have three choices – squeezed (usually called full height or anamorphic), centre punch (which crops the sides of the 16:9 image off) or letterbox (which adds the black bars top and bottom).
If you want a letterbox, then be aware that you are not using a lot of screen real estate and therefore resolution. Effectively you are creating a 720 x 405 signal inside your 720 x 480. The best way to deliver for broadcasters is full height. Both DVD’s and set top boxes allow the home user to set up their box to play correctly for their monitor, so it will create a 16:9 letterbox on a 4:3 TV or play full screen on a 16:9 monitor by stretching the squeezed image back into the correct aspect. This is a function of the TV.
If however you decide to supply a 4:3 letterbox, then approx 40% of a 16:9 monitor will have black bars (both top & bottom plus pillar boxing on the sides. If you want your 16:9 HD program to end up as a small image on a proper 16:9 monitor then go for the letterbox.
As this has been dealt with many times, I suggest you search the forum for the various formulas to create a letterbox in FCP. As for frame rates, I work in PAL (thankfully) but I understand NTSC DV is only available in 29.97, not true 24/ 23.976.