I recently finished my charger and installed a rebuilt 727 bb but after driving it for a few days I noticed a tranny leak. When I put it on stands I found that the rear trans seal and bushing had come out and were laying next to the u joint. My question is whether the trans can build up pressure to push them out or possibly they weren't seated correctly.  

Maybe your output shaft tail bushing was bad to the point where the yoke wobbled a bit and popped the seal out.
There is no real fluid pressure in the tail shaft.
Just drop the drive line and speedo cable/gear, then unbolt the tail housing and take it out & use the new bushing to remove the old one while at the same time installing the new one.
POP a seal on the tail and yer ready to put the tail housing with new gasket back on.  

I've never heard of the seal AND the bushing being pushed out. I would defintely pull the tail shaft housing and make sure there are no cracks in it before I simply push a new bushing in place. If there is a crack in the bushing/seal area, the vibrations of the drive shaft turning could be enough to allow the bushing to work its way out.