The important difference between ST and LT is that an ST is calculated in its maximum load for a lower speed so more deflection allowed wich gives more load to carry at same pressure.
I once calculated it for almost the same sises and maxloadpressure for ST and LT and the ST had 15% more maximum load.
But its always better to give the ST the same deflection as an LT would have. And then even for tandem-axle use the maximum load for dual load ( so 4 tires on one ( virtual) axle).
Then even add 10% to the load on the tire for things like , pressure loss in time, unequall loading over the tires , misreadings of pressure and weight scales, etc.
Then the story gets different for your tires.
Calculate for GAWR wich is 6000x0.9= 5400 and add 10% for reserve= 5940 lbs ( 0,1 is for tonge weight minimum allowed in America).
Practice is that Trailers are always loaded to their maximum.
But weighing and then preferably per wheel(pair) makes it shure.
Old tires then 7040/1.15= 6122 lbs maximum load together of the 4 tires when they would be LT for single load. Then substact 7.5% for making it dual maximum load as is done in Europe ( America sometimes even 9% substraction) gives 6122*0,925= 5662 lbs 4 tires together dual load LT.
Then you need about 5940/5662=1.05x50psi=52 psi, so higher then maxloadpressure wich is 50 psi but allowed up to 10 psi by TRA.
If you now take the new tires with 8160 lbs maximum load for the 4.
Then 8160/1,15*0,925= 6528 lbs maxload dual LT for the 4.
Same sum 5940/6528= 0,91 * 65 psi= 59 psi to give it a save deflection of the tires, asuming D load also are ST.
With my caravan trailer tire pressure calculator you can calculate it more accurate but these will be the advice pressures.
With these pressures , asuming the load is realy to the max, still acceptable comfort and gripp.
