How much safety I can tow. Please help check the numbers!

Don't worry about your GVWR, that generation of Dodge had very low ratings. The frame is the same frame as used on a 3500 with a 11,500 GVWR.

With a loaded truck look at the rear axle, the difference between loaded and 6000lbs is how much pin weight you can support. You can safely tow anything you can support with a 6000lb rear axle.

I would consider a small trans upgrade to internals and cooling thou, maybe a little more if you plan on adding any power.