Strangely my 2018 Ram DRW bed tailgate opening is narrower than the 2016 Ram SRW I had. I had plywood cut to fit in the bed including behind the fender wells to support the Bigfoot wings. I had to cut an inch off the width to get the plywood in the bed opening. My rubber bed mat has the same issue. I might have just over an inch clearance in the tailgate opening with the camper in.
Also, I had to put a couple sheets of about 12" x 48" x 1/2" plywood against the front of the truck bed to move the camper back so I can open the rear doors without hitting the Fastgun tiedowns. I'm thinking I have a little over 49" between the wheelwells. I'm not really sure what you mean by tapered though.
My bigger issue is my tiedowns on the Bigfoot are too far inboard. My Fastguns will rub against my fenders. I just posted a thread about that. Even with an eyebolt in the Torklift tiedown extensions, I'm against the fenders. I talked to Torklift about longer extensions and they have no solution. I have a call in to Bigfoot about re-locating the tiedowns. I may have to put eyebolts on the Bigfoot tiedowns and the Torklift extensions. I know I shouldn't pull sideways on the camper tiedowns, but I might anyway. Another solution is to have a weld/fab shop make copies of the Torklift extensions, just a few inches longer.
Bottom line...I have just enough clearance in the tailgate opening and enough between the wheel wells. No issue with the overhang hitting the tail lights. No issue with the cabover to truck clearance, but I have plywood and a rubber mat. My only issue is the tiedowns.
Are you a dually or SRW?