360 Thermostat and Housing?

75/76 is your cut off year. Ask to see one from a 73 and a 79, for example. If you try to put the small one, in the big hole, it literally falls in.
And the big one in the small neck, is obviously too big.

The aftermarket intake companies blur these lines, but not difficult to figure out. Rumble is guiding you down the right path.

Furthermore on oem waterneck housings, the bolt pattern changes with thermostat size too, so they don't interchange.