Cooling issue, but this time it's to cold...

OK on the non-Magnum.... if the head gaskets were flipped end to end then the rockers would have oiling problems. As I recall and from the Edlebrock pix, there should not be a recess in the manifold but it's sure worth checking. We have the same manifold with a stock type t'stat housing and the t'stat works fine.

It pretty much seems like the t'stat is being pushed open by pump pressure alone, possibly as the element heats up a tiny bit. Time for a Stant, please. There HAVE been reports on this site in the past of multiple bad t'stats...... and the big box store stuff is always suspect.

And the temp will go up and down some with airflow and engine load changes with a properly operating t'sat; the t'stat HAS to have coolant temperature changes to change its opening and do its job. It should be in fairly narrow range with normal cruising around, but it will be there.