Some of the M1 manifolds used a smaller thermostat, maybe the same one as used on Magnum engines.
Your new manifold is probably specced for the one used in V8s from the 1950s until the Magnum came out.
Yes I think you’re right. Because when I put my old thermostat housing on the M1, it was too big and I needed to get one for a magnum, so does this mean that the larger one is standard for LA intake size?