INI,
This pick is of RaptorAddicts truck which carries a constant additional weight of 500-600lbs in the bed of his truck at all times. He opted to go with the HD springs for this very reason.
This pic was taken with a full load of fuel,his rear bed cage with two generals, tools and jack.
We have had up too 1400+ pounds of leafs in the bed of our truck and she popped right back up after unloading. Last time out testing I had two tires, full fuel and tools in my truck and the extra weight really helped the balance of the truck. The rear-end felt even more planted and was more compossed than it normally was without the extra spare.
If you could take messurments now that you are back home and let me know that would be great. If you take lots of long trips and are carrying 400lbs for long periods of time you might be the perfect canidate for the HD versions.
The whole purpose of the Standard Duty spring is for the client who has an empty bed or very minimal weight 90% of the time. This doesn't mean you cant load it up from time to time, just know, the design objective is geared more towards this application.
Lets put 400lbs into perspective. That is almost throwing the equivelant of putting 4 37inch BFG Projects in the bed of the truck. 2" of drop is right in line with what National Designed the standard duty for.
Shoot me a PM if you would like to discuss the HD version.