Here is how I modified the drop down USA Trains Hudson front coupler so that it has a working G scale Kadee #904 straight shank coupler. I drilled a 1/8" hole 3/8" from the end of the shank and sandwitched it between two 3/8" flat brass washers. Using a longer version of the existing steel bolt, I tightened the bolt snugly, but just tight enough to allow side to side swivel. Then I oiled it. A smaller hole was drilled near the knuckle for the chain fastener. The coupler no longer functions as a drop down which is unavoidable with the large Kadee G scale knuckles. Only the smaller Kadee 1 scale (1/32) straight shank couplers can be used as a drop down.
This arrangement has a 22.9 degree range of motion (11.4 degrees left or right). It allows me to push or pull my other Kadee equipped cars without derailing in my tightest curves (R = 5 ft.).
I used a somewhat similar arrangement for the Triplex front coupler (left side in photo below. It is coupled to the protocoupler on my USAT Hudson tender on right):