It's probably not a co incidence that the weight of your boat, plus max load makes 8,100 lb, which equals 3,674kgs.
Now your tubes give 3700 liters or 3700kgs of bouyancy.
With the amount of water displaced by the fibreglass thickness of the hull, which let's say 500kgs, means you have a nett bouyancy of at least 526kgs , in fresh water and 540kgs in salt water, or about 1,160lb in fresh and 1,190lb in salt water.
You don not need to consider any weight of water inside the boat, it is immaterial.
Basically you could do is push your boat right underwater, and if you let go of it, it would bob up to the surface, unless you put another 1/2 tonne of weight in the boat.
As you say, you are not gonna sink, as long as the tubes are inflated, and attached. Period.