I just realized the couplers that came with the Atlas Adjust-a-coupler set have shanks that are approx 1/8" longer than the Kadee #804. They fit fine in the Kadee pocket and work well with Kadee springs. Might be just the answer for a long car that has interference issues.
I have some data to share in case there's others out there in trainland that are doing this.
I have 2 Atlas single-sheathed boxcars with cut levers on each end. I attached the Kadee pockets as close as possible to the endsills and used a 1/8" styrene shim to lower the pocket to specs.
I was going to use Kadee couplers in the pockets, but found that the cut levers interfere with the Kadee knuckles because of the short shanks. I HAVE to use the long-shank Atlas couplers unless I (1) remove the cut levers or (2) add more shim to lower the pocket to less than 11/16" centerline to top of rail or (3) move the coupler out more from the endsill.
This combination of Kadee pocket and Atlas coupler (with Kadee spring) works fine on this type of boxcar.
I have an Atlas Woodrack car that does not have cut levers and the Kadees work fine as is.