Here is a short excerpt from a test of the Mark II (Popular Mechanics, September 1956):



Otherwise I agree with Shelly: if the engine is original, the valve seats demand the lead.

The engine of 2503 got hardened valve seats due to the overhaul so that I can use unleaded fuel.

The ignition timing of 1966 is set to 12 btdc, no knocking because I refuel 100 Octane gas. Fuel economy is 13,4 mpg.