I built a building from Moore with a 24' and a 36' door on the east side.  Center post on wood post worked fine--for about a year.  Then freeze-thaw started working on them, had to adjust the doors every spring to get them to work, and in the winter sometimes had problems as well. 

Neighbor put his guide on an I-beam and buried it in cement, I later bought his land with that building.  We've never had a problem with that guide so that might be an option, or we are just lucky.  I don't know how deep he buried the I beam, but it must have been well below the freeze level.

Personally, if I had to have a sliding door in order to enjoy all the bird problem they create, I would consider pouring a 3' footer under the door as someone suggested.  The newest building has overhead doors with openers, and it is such a treat.  Wish we would have been able to afford them years ago.  Overhead is really the only good way to have a door in the shop, or even in a machinery building, for that matter.