You've got a pretty good plan. I would not go wider than the existing frame. The tires would look odd (IMO), sitting outside that 76" width; and you are limited to less than 80" before you need increased lighting and markings:
http://www.nhtsa.gov/cars/rules/standar ... rpstr.htmlIf you leave the chassis as is, you can either build boxes around the tires and have a raised bed with storage underneath; or, you can do as you show in your drawings and use a queen sized mattress (60" wide) with a little space around the mattress. At least, that would make it easier to make the bed. My preference would be to keep it as simple as possible.
I love to see pass-throughs on teardrops. They make perfect sense to me.

From what I see, I'm assuming that you are planning on camping in the USA, probably in the Southern states, as it looks like you only plan on visiting campgrounds with electricity plus you're trying to fit air conditioning in. Also, I don't see any 12V outlets. Be aware that, if you plan to camp out west, especially the northwest, many campgrounds are off-grid.