Can I increase payload of Penske 16ft truck?

I’m not certain with GM stuff. In the E-series, from 350 to 450 is just the springs and a way harsher ride. You might swing by a GM dealer service department and ask.

 

Makes zero difference what you do to the vehicle, the GVW cannot be changed by just changing parts.

Just go buy the truck that can handle what you require and go to work lol.

 

You would need to have the line sheet sticker from both trucks and compare components.

Subtle differences in brake capacity and suspension components could be invisible to the eyes.

OP has received much valid information above.

The warnings about the differential prove one caviat:

General motors runs on cost basis accounting reducing everything to it's lowest possible cost while still meeting 99% of the 'promised' GVWR to keep the feds off their back.

Going above the GVWR may work for a while but will have maintenance issues if not safety problems of all systems.