It seems to me that you have more payload capacity than quite a few half tons of the era. With a curb wt of ~4300 and a GVWR of 6000, You can almost haul a ton legally. But that would put quite a bit of weight over the rear axle and I'd be concerned with the rear axle rating. It should be on the door sticker as well. It wouldnt be hard to be under the GVWR of 6000, yet be OVER the RAWR.
I agree that you really should have a bigger truck, but just how often do you tow?? With a properly set up trailer, and brake controller, your going from a 2 horse trailer to a 2 horse trailer that only weighs 1k more??? And you say currently it tows fine??
I also agree something is amiss in your GCWR# of 5500lbs. The GCWR "should" be somewhere around the GVWR (6000#) + whatever you can tow. You can make whatever alterations, gears, springs, etc you want. But when it comes to legal, they go by the door sticker. You could put that dakota body on a 5500 if you want. But you still cannot "legally" tow more. So keep that in mind too.
Quite a few half tons out there tow 8-9k+. And a dakota isnt much behind a half ton. So towing 5-6k I dont think is a stretch. Just make sure you dont overload the rear axle, make sure you have good brakes/controller, and make sure you arent running P tires on the rear.