On the old SHS coupler, the hole on the coupler shaft was also larger than the hole in the truck. The larger hole fit over a slightly raised boss on the car's bolster:
Here it is after fitting it to a Lionel/Flyer Hopper using American Models trucks. No glue, super or otherwise was used. The only thing holding everything together is the truck screw:
I don't have an MTH hirail car to compare against, so I can't study how the coupler might have cause to flop around. But, there are super glue debonders available that will enable you to remove the coupler from the truck at a later date if so desired.
Rusty
