Assuming for a moment that you do roll out loaded at 8,400# you will have roughly 1,200# of tongue weight (including the WDH). A trailer that long needs to have at least 13% tongue weight. If you roll out with only 840# of TW ( which by the way is just not going to happen), that is a sure fire recipe for sway, especially on a 33'+ trailer pulled by a half ton truck.
IMHO: You are needlessly pushing the envelope but as noted above if you stay in the flat country and don't go to far you "can" do it. Consider too that with 1,200# of your 1,600# of payload gone to TW that only leaves 400# for passengers and "stuff" that you can put into the truck and that 400# will disappear very quickly.
As always.... Opinions and YMMV.
:C