You talking legal cargo weight or the max the truck will handle or the most it will handle without risking trashing the springs?
For legal weight, just take it to a local supply yard or scrap yard that has a scale and ask them if you can put your truck on there an get a weight, subtract from your GVWR and you have it.
The most you can put on the truck without trashing the springs would probably be no more than 1,000#
The most the truck will handle.... well..... I had a ton and a quarter on my Ranger before.... I also needed new springs after a couple times of that.... so I custom built some heavy-duty packs after that until I got an F-150 to handle regular work duties (and I put modified 3/4 ton springs under that, lol).
