1680lb payload:
- 200lb hitch
- 200lb cargo in the truck (I'm being generous as I've yet to see a 5th wheel pull in with the truck completely empty.
That leaves around 1280lbs for pin weight. Assuming 20%, that allows for around a 6000lb 5th wheel...except....
That's the fully loaded weight, remove 1000-1500lbs and you are down to around a 4500-5000lb 5th wheel if you look at the dry weight.
(run the numbers as some 5th wheels are closer to 25% pin weight reducing the available size substantially. Also, you will be very limited on other things you can carry in the truck.)
There are some very small 5th wheels (mostly older used models) but your selection will be limited.