Dave,
These old engines tended to run their heads kind of hot. They also were real champs at "carboning up". This combination leads to "Hot spots" of glowing carbon which tend to act as glow plugs and keep things running like a diesel. Getting your idle speed as low as possible, consistant with smooth running, will help to minimize running on. So will letting the engine idle for a minute or so before switching off. The use of premium grade fuel also sometimes helps.
The only other solution would be what the Brits refer to as a "decoke", wherein the cylinder head is pulled and all of the deposits are removed from the valves (great time for a valve job w/ hardened seats and bronze mangenese guides, etc.), the combustion chambers, and the piston tops. Thereafter, steps can be taken to slow or prevent the build-up of further "coke".
Run fuel system cleaner through the system now and then, for instance. There's another old trick that actually works, for the most part. Get the engine good and hot with the air cleaners off. Run the RPM up to about three thousand and mist a couple of shots of plain water into each carburetter. DON'T do this on a badly carboned up engine. Just do it to maintain one you know has been decarboned (decoked).
Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon