Just in case anyone else is looking, since it takes some digging to find the proper screw in Kadee's instructions if you're not familiar with them (of if you somehow wound up with older-style packets like the ones I got from Red Caboose, that don't mention the "recommended tools" on the back like the newer ones)...it's a 2-56 by 1/2" screw you want. I mistook my measurement of the box thickness as 1/8" instead of 1/4"
In a pinch, your local (or semi-local) Radio Shack carries an assortment of 2-56 machine screws in various lengths from 1/4" to 3/4" that will fit the mounting holes in the 805/806 coupler box. The part number is 64-3010.
If anyone runs across this post while looking for a coupler solution for the K-Line K672-7501a Kennecott Copper ore car 4-pac (it was designed to be run with a handcar set rather than the Plymouth switcher), I'd recommend starting with the Kadee 806. The regular production ore cars have clones of the Kadee 805, but the slope sheets and end sills were notched to accommodate their length. The Kadee 806 has a shorter draft gear box that won't interfere with the slope sheets, but you'll still need to trim end sills and screw boss so the coupler sits at the proper height, as these cars were designed to run with drawbars within the handcar train, or with optional dummy "lobster claws" on each end of a drawbar-linked set within a regular train.
---PCJ