I have heard this before -
"I went and got my plates. I have it registered for 42K, now I can carry heavy cargo."
I asked "what's the truck rated for?"
"ah... I don't know I think 32k"
The thing about registration doesn't give anyone a pass on the actual ratings of the truck. You have to be careful if you register it at say 42k and your truck is rated for 32k, when you hit the scales with a single axle truck and it tops 34K total weight (21k empty weight plus 13k cargo weight), then there may be an issue with the people in the station regardless what the registration says.
Get a second axle or a drop axle to carry the weight.