If this was your original:
Red = edge of monitor
Yellow-Green = edge of frame
0.)

= 848/480 = 1.766 ~ 16/9 = WideScreen
You've got 3 options:
1.)

crop(0,60,0,-60)
= 848/360 = 2.355 ~ 2.35/1 = CinemaScope (aka x-wide)
2.)

crop(0,60,0,-60)
addborders(0,60,0,60)
= 848/480 = 1.766 ~ 16/9 = WideScreen
(Although the picture is Scope, the frame itself is WideScreen so it fills the entire monitor)
3.)

addborders(0,78,0,78)
= 848/636 = 1.333 = 4/3 = FullScreen
(oh the irony of FullScreen videos on WideScreen monitors...)
There is a big difference between Borders and Letterboxes. Borders add black around the perimeter of your image. Letterboxes turn the edges of your image black. Assuming most people have widescreen monitors these days, if you simply add Borders to the top and bottom, you'll just end up with the appearance of a rectangle in the middle of the screen (like #3), and I'm not sure if that's the look you are after. However, Letterboxes effectively do the same thing as cropping then adding borders equal to what was cropped off (#2). While this does do what you are looking for, it is pointless since it looks the same as cropping, so you might as well just crop to CinemaScope (#1) which is about as cinematographic as it gets. I'm pretty sure that's what they were going for when they made The Animatrix (
example)
But it's unlikely your footage is scope to begin with, so you'll have to crop to 2.35:1 or something close to it (like #1). That way the blank space looks like you've got black letterboxes (#2), without actually encoding them into your video. As an added bonus, your file sizes could be as much as 25% smaller. It might help if you edit at 16:9, but
overlay letterboxes in your editor so you can see what parts will get cut off. You'll have more control this way. And if you want to actually use the letterboxes for something (like text), at least you'll have them. You can always crop them later.