First, let me say that I pull a 22' travel trailer that weighs about that same weight. The Explorer loaded is about 5500 lbs. Check the limits for your model, equipement and year and you will likely find you are over the trailer limit as well as the GCVWR. Mine tows fine but expect to change the trans fluid every 30k miles or so. I would highly suggest the trans temp gauge and a trans fluid cooler.
Second, I am suprised you fond a class V hitch that fits your Explorer. 1000 lbs on the hitch, even a wieght distributing one, will likely make your rear set low. Unless you crank the rods on the hitch way up. Either may make handling a little squirrely. You told us the maker of the hitch, what model. There are a few of us that this would interest.
Third, I have no problems stopping. Good controller set up correctly keeps it balanced. If the brakes on the trailer are not capable of providing for its complete stopping needs, the trailer brakes are not adequate.
As for going, if you are satisfied with the performance of the 4.2, you will likely be satisfied with the 4.0 SOHC. Remember, towing over weight recommendations will wear parts faster, decrease service intervals and require additional modifications to do it safely and minimize the wear and tear.