The GVW of the 338RETS is 10,950. 13% of 10950=1423 before adding weight of hitch.
With that in mind you should be OK. I would invest in a Sherline scale to ensure you are within limits at all times
Try to keep your loaded weight around 10,500.00 and you should be OK. Keep in mind when all is said and done you will be hovering around the 1400# limit.
Your 3500 should have the capacity but you may want to double check your receiver limit as well.
You can control your tongue weight to some degree by loading heavy items behind the axles. Using the Sherline scale you can adjust your cargo accordingly to maintain the correct TW.
