I found this:
Gross Vehicle Weight
The gross vehicle weight (GVW) is what determines how much exhaust flow will occur during ASM test conditions. Engine displacement does NOT have as much effect on this as GVW. A small engine will operate under a wider throttle opening than a larger engine so that both engines will have similar exhaust flows. In fact a small engine in a heavy car will be operating under higher combustion pressures and tend to have higher NOx emissions than a larger engine in the same car. Ignoring vehicle weight when selecting a catalyst can result in ASM test failure. Using a converter that is rated for up to 4000 lbs on a 3999 lbs car is asking for trouble. EPA
certification only requires that a catalyst be 70% efficient at oxidizing CO & HC, and 60%
efficient at reducing NOx. Original equipment catalysts are sometimes sized to achieve over 90% efficiency. When in doubt use a larger catalyst from a quality manufacturer and a reputable distributor.
My thoughts is that the more weight your pulling (up to the rated GVRW of the truck), the more exhaust flow you'll have for a longer duration. Using lower GVRW rated cats will not allow the truck to pass an emissions test or the new lower GVRW cats will fail prematurely.
If the cats you are looking at are a complete head pipe and cat assembly (bolt on), then the entire assembly has a GVRW rating. It should equal or surpass the GVRW of the truck.
Cutting off the old cats and welding on new cats to the old pipes, will require that the combination of both cats equal or surpass the GVRW of the truck.
Example:
Truck GVRW=10,000lbs.
Pipe and dual cat assembly must equal or surpass 10,000+ GVRW
Individual cats welded to old pipe-each cat must equal or surpass 5000+ GVRW
I hope this helps.
