I remember him saying that adding a long bed would mean more complication by adding more chassis to update....
The post by VW is correct, Gabe said in the update speculation thread after the crew cab pic was shown that adding a long bed would require too many chassis to maintain (Imagine changing parts for 12 chassis every time the D series gets a single update or new part or something). I personally haven't seen any mods for it that look good IMO, but there are a couple out there. A couple people have made suggestions for a long bed on the existing frames that seem reasonable to me, but I haven't seen any further developer responses to those.