At 400 hp you have taken yourself out of the MDT category. Pulling 20K trailer you won't need 400 hp unless you hang out in the mountains. So I guess you first need to define your pony needs.
Also when you say you want a automatic, are we taking a like automotive auto or auto shift? You should check out the new FL M2-106. You can spec it with Allisons 6 speed world tranny which is very smoooottthhh and a 300 hp motor. This is basically the same setup as most pushers.
When your looking at MDT's over Class 8's your luck will run out much quicker. MDT's are generally used inner city and beat, not saying class 8's aren't hammered on, but I think it is easier to spot a bogus 8 than MDT. Also if you go used, check that rear end! Most inner city trucks are setup up to run 55-60 mph tops.
Here is a sweet ride --> http://www.truckpaper.com/listings/f...earch%3DSearch