You likely don't need the slider hitch, but you will want to keep a 6" minimum of bed rail clearance. You say about 6"s, so is that just eyeball measurement? If you actually have a measured 6"s, you would not want to decrease to 4"s by changing hitches. You could raise the trailer a little by going to larger tires/wheels if you have enough wheel well clearance. Adding the correct track alignment system can raise the trailer a couple of inches.
It likely comes down to raising trailer, maybe by adding a subframe, or just towing nose high. You know how it towed with the old truck, so see how the new truck tows in comparison. IMO, 3"s nose high or less for best tow, but you see much worse being towed.
Jerry
