I'm sure I'll catch He== from the DRW fanatics for this, but THERE IS NO MAGIC IN DRW!!
You have twice as many tires, so you have twice the TIRE load limit. You have not changed the rear axle weight rating and therefore once the total load bearing capacity of the 4 tires becomes more than the load bearing capacity of the axle, the rest is wasted. Stability is a whole 'nother question and I have some Engineering opinions on that, too, but for another day.
The rest of a 3500 truck is a 3500 truck.
To the OP,
Load it up. Hook it up. Run it across a set of CAT scales and find out what your actual weights are.
