I have a 2270 type 4 motor with a Mallory distributor, dual 45 dellortos and headers. The #1 cylinder head runs appreciatively hotter that than the other cylinders.

I checked the valve clearance and they are with spec, timing is correct and the AF is within the range.

Any suggestions on what might be the cause or to check?

Off the top of my head I would make sure the cooling fins are clean and clear for air flow. They can get gummed up with oil and dirt. Also make sure the cooling flaps are working properly. They should fail in the full cooling position but can stick closed.

Sounds like you have a clogged jet on that cylinder causing it to run way lean.