Stop the guessing. Hook up and hit the truck scale.
You could simply weigh the whole combo and get your Gross Combined Weight, which is the be all end all.
Then break it up to get the truck weight and weight on each axle.
Payload is a derived number from Curb Weight (actual, as-is scale weight) minus the Gross Vehicle Weight Rating.
I believe you will be at or near your trucks rated tow capacity and perfectly fine to tow with. It will simply be more work for the engine with the tall rear end gear, which is the only thing that lowers the tow rating. So mechanically, you are not "un-safe".