I think if you estimate the lift device, framework addition and pto/pump equipment at around 1000-1500 lbs, on top of the curb weight for the 6409 Cab, platform and stake rack (5,585 lbs), you should be in the right ballpark.
Definitely go to singles in the rear for loading and road clearance purposes, but also consider Bob's mention of having a trailer of the appropriate length for the load and tongue weight balancing. Err on the side of caution for yours and everyone else's safety.
