Tom, I did mean the bushings would need a conical seat. Apparently that is not how the current brass replacements are made and that is why I suspect the result would not be worth the effort.
The link coupler trucks are metal on metal and that has a relatively high coefficient of friction when dry. I can get mine to roll freely by cleaning them with electrical contact cleaner and lightly oiling them. Unfortunately that lasts less than a week. Metal axles on Delrin plasic would be much lower friction in the dry state. I think that is how the new trucks are made.