If it sits into the machined area and does not fall into the opening it is the correct one. All the 1960s-1980s used the same stat. Maybe you've got the wrong thermostat (water outlet) housing???
Rock Auto Parts or any auto store that carries Four Seasons brand #35504. Any NAPA store can cross the Four Seasons number to their brand. Rock Auto price is about $31 plus shipping.
If you look at the "H" on the gauge as a goalpost and the needle stays between the uprights you are okay. The radiator pressure cap raises the boiling point to about 240 degrees.