With that (what looks like) a diesel tank in the bed of your old truck, I don't know why you would even consider a 6' bed truck ... . you NEED an 8' bed truck. With that tank you will have only 6' left for stuff and the 5th wheel/goose neck hitch.
Save your money and get a Quad cab 8' dully like you had before ... or ... if your towing quite a bit of weight all the time and don't need 4X4 consider a step up to a used 4300 International or Freightliner with a crew cab ... everything you need to stop a large load.
Save your money and get a Quad cab 8' dully like you had before ... or ... if your towing quite a bit of weight all the time and don't need 4X4 consider a step up to a used 4300 International or Freightliner with a crew cab ... everything you need to stop a large load.