Box Truck w Airbrakes....Concerns weight capacity

I have a 26ft box truck with airbrakes. I currently have it rated as a non-cdl truck @ GVW 25,999. I have gained new business that will likely put me over that weight. I currently have a CDL-B which allows me to drive a vehicle that can handle that additional weight.

Assuming I adjust my rated GVW for my truck above the 26,001 threshold, My questions/concerns are....

How do I find out how much weight my truck can handle before I blow out my axles? Meaning....is my truck built to handle the weight of CDL loads but the only reason it becomes a non-cdl truck is because I rate it that way?....OR are CDL rated vehicles built differently to support the additional weight than a non-cdl vehicle? I'm assuming there is a "carry-load" somewhere with my truck, but I can't find it out....and if I were to "alter" my vehicle, how can I make sure I'm doing it safely and under proper regulations (ie adding axle/changing box size/etc)?

Any information is appreciated....or a direction through FMCSA/DOT or other resources. I'm not really sure where to turn to, to get the CORRECT answer.

Ty in advance....

 

Meaning....what I disclose of the weight I will carry up to (ie 25,999)

 

Correct

 

My truck weighs 17k empty...

The GVWR is 25,999
The GAWR is: 10k front axle & 19k 2nd axle

With my 2 axles (GAWR) able to handle 29k, is that irrelevant to the 25,999 GVWR?

Attached pic if it helps.