I have always heard the old saying about making sure your towing vehicle weighs more than your trailer, and understand it, but not sure I agree with it. I think that old saying may have come to live in the days before people had trailer brakes and if you stepped on the towing vehicle brakes, the load would "push" the towing vehicle. That is one ride you won't like, BTDT!!! I tried to describe it to a friend who towed a 26' Bayliner with a '79 Corvette. He was lucky and never learned it first hand, but the boat trailer did have good brakes and he kept it below 30 mph.
Before I condemed the passenger tires, I would look at the sidewalls and get the load rating from them and figure out what kind of weight your going to have on them. With a good trailer, most of the load isn't on the truck axles, although you do have to put some up there. Do the math, see what adds up.
As far as driving the tractor 50 miles, that would be no fun at all, but to each his own. If the 50 miles is all Interstate highways, at 55 mph, the tires might get a little hotter than backroads at 30 mph. Consider that also, along with the age of the original tires. The 5K load rating is probably going to be for the tractor and it's trailer, plus all the implements and passengers, so you start eating into that 400 pound buffer rather quickly. Ask me if I have ever towed a trailer over it's rated capacity and I will look you straight in the eye and lie to you!! Never should you do it, but I am not saying it isn't done. (by me!) The chances are you will be fine, but IF something happens, such as a someone pulling out in front of you, etc, and they hire an attorney to fight you, you may loose on the grounds that your vehicle was "severely overloaded" (Doesn't that sound bad?) I knew of a lawsuit where the guy was really close to the weight rating and hit a kid on a bicycle. Nothing the driver could have done, but he was overloaded and the courts blamed that on him. His liability in the suit ended up being 60% instead of 0, which cost his insurance company a small fortune.
Bottom line, we pays our money and we take our chances. Good luck with yours, whichever way you decide to go. Either way, be careful.