Use a Kadee 745 (metal coupler/plastic box).

Use 2mm x 8mm screws and loosely secure the box to the car using the center hole, then fudge the inboard screw in place. When the threads catch, evenly tighten both screws. The plastic box has just enough flex to line everything up. You may have to use a 1/16" shim to lower the couplers to the correct height. I've done several Atlas cars this way and they've worked great.

If you want to use the metal box. Which I’m guessing is why your switching out what’s there. You can try drilling the mounting holes in the Kadee box with a #43 bit. The Kadee’s are designed for a 2 56 screw. Which is slightly larger than the Atlas 2mm. Between the smaller screw and the hogged out mounting holes. It should go on now with a little more wiggle room. Just follow Matt’s method of alternately tightening each screw. It also is best to use a Phillips head screw. Catching the second screw. You have to keep some tension on  the screw to get it started going straight. A lot easier with a Phillips head.

@Diego posted:

I recently bought a 68’ flatcar. Took a kadee off but now I want to put a kadee back on. I bought 805s but those didn’t work. What kadee should I buy?

Why not put back the same Kadee that you took off?

Kadee has a very good website of all the available O-scale couplers.

Edit/Add: 7/10/23, 6:30 AM.  There is/or was, a Kadee height tool/gauge, (812, O gauge), that can be used to install all couplers at the same height.

I use a #42 drill to hig out the rear hole on the box to slot.    You only need about 1/32 inch - a little less than the thickness of the rim around the hole in the rear.    Then I use 2x8MM screws to mount the coupler box.    This way neither screw is being pushed sideways in the hole and messing up the threads.    this is especially important with plastic underframe cars. 

If  your problem is coupler height, Kadee now has over mount and under mount couplers that raise or lower the heard about 1/16 of an inch.

The other thing is that the patent on the original KAdee 805 must have expired.    the newer Atlas cars I am getting have a Kadee knockoff coupler that fits in the Atlas box.    The coupler head looks like an 805 but the seems to be built to less accurate specs.    They do work.   I find the springs too strong however, so I replace both the spring on the coupler head for the knuckle and the one in the box with Kadee springs.    They seem to work pretty well then.     Atlas probably sells them separately too.

@mwb posted:

Why not put back the same Kadee that you took off?

I broke it when trying to remove it. Since I have to wedge a screwdriver to remove cover

@Diego posted:

I broke it when trying to remove it. Since I have to wedge a screwdriver to remove cover

Hmmm......never had to do that with a Kadee............Atlas, yes.

One minor issue is that the end of the Kadee box sticks out from the end sills by about 1/16". Easily filled with styrene if you're a stickler but barely visible under most circumstances. I haven't filled one yet and no one has noticed.