I've never seen a 20 foot dropside bed on an M927 or m928 (well actually I've never seen one on an 800 series long wheel base truck either).
You may also have some issues with the bed sticking 6 feet over the back of the frame. The M927's and M928's have a reinforced frame to help support the extra weight.
I just did the opposite, I put an M923 bed on an M927 and put a 5th wheel behind it.
A friend of mine just bought two M927's from Camp Shelby and he is parting one out. He will sell the bed if you want to come get it.
PM me for contact info.