[Coley]

The GVW plate rules the day for any grievance or traffic violation issue.
To make sure you don't over load the truck...you will need to specifically weigh your truck. I recommend a relatively empty weigh in (log this) ....and then a full load weigh in.
Note that every single one of these trucks weighs a little different so don't go by any stock or generic literature to 'guess' the truck weight....literally go weigh it.

Now, the GVW rating on the trucks encompasses more than just the spring options.
The tires play a role as well as the brakes, the frame, etc. The heavier trucks with the higher GVW got most of this stuff often built into them....such as the camper special trucks.

I have been through this cycle repeatedly on the half tons trying to figure out why the capacity never seemed that high based on the GVW plate (ie: 5000 lbs)....and I did this because I wanted to carry a small 8ft overhead camper. I did this successfully, but I was always over the original GVW of the truck.

Now, from GM's standpoint they are always going to under rate the trucks....always. This is their factor of safety to compensate for the guys who always overload their trucks....and you will never know that safety factor number.
However, in defining the GVW they would base it on the weakest link in and under the truck...and that is usually not the springs.
On the original 1/2 tons it was the tires. Those old technology bias ply tires were lucky to carry 1500-1600 lbs each...which would (on a good day) give you a maximum of 6000 lbs for safety...and of course GM would de-rate that.

Also, I'm pretty sure that the auxiliary springs did not bump up any GVW ratings...they simply were marketed as overloads to help with load stability and not necessarily increasing any rating numbers....at least this would be GM's safety/liability default point.
Why?....because you could remove them ...and then still claim you were within the GVW rating.
(I noticed this on a 1/2 ton I had that came with the overloads....and I too, was expecting a higher GVW plate rating...but there was no change in it)

Today, with modern tire technology the GVW rating of these original 1/2 tons would probably easily pick up another 500 lbs or so, given the fact that any 15" truck tire today has a rating above 2000-2200 lbs each...for a net safety/capability of 8000 lbs with 4 tires...although GM would never give you personally the benefit of that doubt relative to GVW.

The other thing to factor in is if your springs are original. If so, they have probably lost 500 lbs worth of ability vs. a new spring.
As a result if you want to safely carry bigger loads, get your springs redone and you really see and feel a difference.

My two bits.
Coley