When I started out I intended to do something like what Ben did and just change couplers. But so many of my Lionel 50' flats didn't like The Atlas switches and shorted out. Wheels out of gauge. Lionel sheet metal couplers are just a pain to work on. Decided to replace with Atlas trucks which required a bolster spacer. Came up with a one piece frame extension that extended the frame, spaced the truck, held the coupler and filled the gaps in the end frame. A lip at the top held it in place with the end of the main frame between the frame and wood floor. Super glued the coupler end and fillers to the end frame. Truck mount screw held it together in the middle. Just had to use longer screws. Break the super glue bond at the end and the car can be restored to 3 rail with little damage. A little scraping of super glue and paint touch up.
Made 2 masters out of plastic. One for 40' and one for 50' flats. Made silicone rubber molds of them. Cast a bunch in resin. Did all this sometime before 2012 when I shelved the hobby. Won't be anymore. Just used the last 2 50 footers on the Lackawana piggyback. One of them may have been the master. Did find the master and last 2 of the 4 footers to take a pic of.
The 50' piggybacks were harder to do. The frame opening was narrowed on one end. This required modifying the extension to fit properly between the narrowed frame rails.