You did the arithmetic correctly. The 245/75's are adequate for your rig but don't have much extra capacity. They will have to be run at/near their max load psi, which might mean a slightly harsher ride.
I suspect your axles are actually loaded less than their max (GAWR), though. You may have more margin their than you think. A trip to the scales would be a good thing. Then you will know rather than guess about your safety margins.
If you feel you want extra capacity, you may be able to find the same size tire with a greater load range rating (more weight capacity), or you may be able to go up a bit in size. However, changing the tire size is not something to be done casually. You need in investigate a number of potential problems, e.g. front wheels might hit wheel well when turning, rear duals may be too close together, speedometer & odometer accuracy will be affected, tires may not fit properly on the existing wheels, etc.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL