Came up with this method years ago when 3RS was a baby. First is the K-Line conversion. Using 5/16 X 5/32 brass rectangular tube. Tube cut to length and notched on the end for the wide part of the coupler shank. Hole drilled for a small screw to hold the coupler in place. Then a hole drilled at the pivot end to mount the tube to the centering T bar mount. 2 brass tubes soldered on to align with the T bar. One of the hardest parts was to figure out the length of the tube and coupler so the cars don't bind in the curves. I run an 072 minimum.
The GGD 21" heavy weights were a challenge. The cars came out in batches over a few years time. Combine, coaches, sleepers, diner and observation. Ended up with a 9 car set. Seems like they came out in 2s and 3s.
Challenge was that Scott made improvements with each batch of cars. The truck sheet metal changed. The mounting method and placement of the 3 rail couplers changed. At least 3 versions. The combine with no vestibule was unique.
Used similar method as with the L-Line. Coming up with a spacer for proper coupler height is the trick. 
This was the last car I just finished. Used Evergreen plastic tube on this one. Ran out of the brass tube. Probably why iit wasn't done then.
tThe centering spring is attached to a screw at the end of the brass rectangular tube.
One of the first cars out. Combine. The truck sheet metal changes.
The Weaver headend cars I run with this train have body mounted couplers.
