The best scenario is to match the tire load rating with the max load of the trailer + 10%.
Too low a tire rating and the result could be very exciting situation.
Too high a tire rating and it is a waste of money and potentially a harsh ride for the trailer.
So "D" or "E" rating sugestion becomes a guess not knowing what your trailer weighs when fully loaded with gear and liquids. Or one needs to know what the GVWR, however a 1959
unit did not have that kind of rating when it was manufactured.