How to create a uniformly placed spritesheet from non uniform sprites?

Hi 'United' People,

Okay, I agree, this is not particularly a Unity question. But I am really stuck and although it seems to be an easy process to think of it, I am too confused. There must be a particular way to do it, cause doing it manually would involve too much mathematics with pixels and stuff. Yeah, now to my actual query.

I have created a sprite sheet with sprites of equal width (120 px) but unequal heights (ranging from 162 px to 180 px) in Inkscape. Now I want to create an uniformly spaced sprite sheet so that I can grid slice it at 120 x 180. But I can't seem to do that and I can't seem to find anything to help me out.

Please guide me on this or provide useful links. I really appreciate the help. Thank you.