To be able to better judge where the hopper doors are to be mounted, I decided to install the trucks next. I marked the center of the bolster section into which the truck screw will be mounted, and used my drill press to drill a perpendicular hole. I used the screw to "tap" the hole. The next time I plan on using smaller screws for mounting trucks. The washer in the photo was initially used, but it turned out it raised the car just that much too high. The model's height was perfect without the washer. I used some S-Helper Service trucks, but they are not the ones I plan on using long-term. (Update: the washer turned out to be necessary when I started running the car, because the wheels hit the underside of the frame without them.)
This is the first serious problem with the model. I put the brake lever too far away from the bolster, and so the back of the wheel hits the lever and prevents the truck from turning any more than what is shown in the photo. I wound up having to break off the lever, fabricate a new one from styrene, and glue it much closer to the bolster to allow for adequate truck swing. This also meant installing a new, longer brass wire for the wire that runs between the two brake levers. The photos on this page don't have the updated brake lever position yet.
Installation of the couplers was specific to the HO-scale couplers I use. They come in parts, so I just ignored the coupler box and only used the brass part that causes the coupler to remain centered, and the coupler itself. I found a brass tube in my parts box that matched the inside diameter of the hole in the brass part and the coupler, cut it to length, and then glued the brass spring mechanism and the brass tube to the underside of the center sill of the car using 5-minute epoxy. This then acts as the mounting post for the coupler. I slipped the coupler onto the brass tube and then fabricated a square piece of styrene with a matching hole in its center. This was placed over the tube and glued to the inside wall of the center sill. The couplers now freely swing left and right, but also return to the center position. Depending on which coupler you use, you will have to come up with a custom installation.
At this point, I couldn't wait to give the car a test run on the layout. Due to its (still) light weight, it derailed on one turnout, but ran smoothly from then on.