Okay, but you *think* the payload capacity is 1420 lbs, but it's not really. Just for fun some day, take your truck to a scale and weigh it. Take the trailer too and weigh it all up and do the calculations. You may be in for a surprise.... Better to be educated on your weights rather than flying blind. Not trying to be critical, it's just that weighing TT and TV is a real eye opener. The payload capacity on our F250 is 1,000 lbs less than what the sticker says.... If in fact the scale tickets prove your actual payload capacity is truly 1420 lbs, I will send you a lovely complimentary gift of some sort!!
We have a 3/4 ton truck pulling a TT weighing just 6600 lb but I can tell you that it is pretty easy to overload the truck on payload if the supercab is loaded up with humans, pets, and other stuff and we load the bed of the truck up with a big load of firewood and other heavy stuff. Even a 3/4 ton ain't as BIG as you might think.
Also if you are driving at 70 mph a lot (or are you being honest on this?)? Driving regularly at that speed is going to lead to premature tire failure. ST tires are rated at 65 max., unless the tires have been upgraded. Even at a routine 65, you are significantly maxing the tires out. There are zillions (exaggerating..) of posts on owners with ST tire blowouts and in some cases, serious damage to the trailer. Again, not being critical, just saying it's better to play it safe and treat those tires with the respect they need. The experienced ones drive consistently at 60 mph.