Yes, 40 Deluxe is right. The Merc you have has a 5 lug, 5" bolt circle.
Here are some FoMoCo cars with 5 lug 5" bolt circle, and Most have 15" wheels.
Ford: Custom 1973-1978, Galaxie, LTD 1973-1975, Thunderbird 1972-1976
Lincoln: All (except Versailles) 1960-1967/1973-1979
Mercury: All 1955-1960, Cougar 1973-1977, XR7 1973-1977
Also, Most older 2 wheel drive Chevy pickups had a 5" bolt circle with
the larger center hole, and the backspace should be close,
dependiing on the width of the wheel you use.
Good luck...
.