I am working on a bunch of freight and express car projects in between scenery efforts this week. I took a bunch of cars off the layout to Kadee-ize and also to refresh the weathering. I also set up some new cars as I go through and clean up my freight roster.
Atlas X29s - I removed the 3 rail coupler and adapter and let the truck float. Haven't had any issues with this setup yet. I am using Atlas scale couplers as they bolt right up. I had a few sets left over from my H21 hoppers. I ram jammed a Kadee 805 into one of my X29s since I ran out of Atlas couplers. Doing this with Atlas cars is ok in a pinch.
I found a 50' express reefer under the layout. I force fudged a Kadee 805 onto the standoffs with #2 wood screws from Walthers. The 3 rail coupler setup was easy to remove. Atlas couplers would bolt right in but I ran out of them.
I have a few Atlas USRA single sheathed or rebuilt boxcars that I've never done much with. I used Kadee 805s with some custom made shims. The shims are 5/32" high by 3/8" wide and are made up with balsa strip and scrap styrene. I then chop them to length, CA them to the frame and then drill and tap for 2-56. I had to experiment a little to find the right hole locations to line up the box with the end sills.