My 2 cents.
Your slider is most likely going to keep you from hitting the toolbox, just try some 90degree turns in a parking lot to test. Do it slow and easy and you can see the clearance that you have or don't have.
I have a slightly newer version of your same truck. For me the 5th wheel pin weight was something I had to manage. It would have been easy for me to buy a 5th wheel that was too heavy for my truck. There were two things I had to avoid.
First was no toolbox (I didn't want to give up the weight of the box much less what I might throw in it).
Second was a slider hitch (they were all just too heavy) but at 6.6ft that extra foot in my bed worked out ok when compared to the crew cab trucks that had a 5.5ft bed. For your 6.5ft bed you may be able to do as I did and use a Curt Extenda Ball which basically moves your in-bed gooseball 4" toward the tailgate and then the Andersen sits on top of that.
Wildcat2016-295rsx GMC2015-2500HD-DblCab D'max/Allisn+4x4 6.6'Bed
Maxxis235/80/16E;Andersen Ultimate2+4"offset;Airlft1 WirelessComp;GroundCtrl3;MorRyde RubbPinBox+CRE3000;3-Slidecovrs;Champion3100w/DIY ExtndRun;10'Portabote+5hp;Furrion48cam
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