Shorts are a pain, in a couple different ways. You never know what will work until you try it. Most of the good ones are hideously expensive.
Now that's out of the way... I mostly use Boure Pro.
http://www.boure.com/1100.html
They will make them to your size.
I have a Performance Ultra, it's nice, but I've never tried to do a really long ride in it. It's prob fine.
One last thing... I use leather saddles, either a B17 or a Titanico. Superb saddles that if setup properly take the drama out of finding shorts.