Ninja UV will straighten them all horiz and vertical. Can get it on CreativeCrash. This is the easiest answer to your problem. You can do it the way Jay has stated. But this tool will get the job done and free up time in your day to do other things besides staring at a computer screen.

I am not here trying to sell this product. I get no money in saying this. Just telling you what I use on a daily basis to create straightened UV's. It also comes with a function to equalize spacing on the map if you are worried about pixel perfect stretching and want to fix it fast. This is a tool I use constantly with Roadkill (Free Program/Script). I hope this helps you and makes you a better artist.

Ninja UV Maya Script - YouTube