I put a Flyer coupler on a 390T and pulled Flyer STD gauge cars with it and my 390e. The 390T had a coupler pocket, and if I remember, the hole was a little small for the Flyer coupler, but I was able to modify it without too much trouble to make it work. The 390T has Lionel 500 series trucks, which are the correct height for Flyer.
Are you talking about STD gauge or O gauge? I assume STD gauge, because I don't recall MTH making any Flyer O gauge.
George