If it is a carb model, then unlike an injector set-up, it is difficult to stop the fuel continuing to flow in if the engine is still turning and the pistons/valves are still pulling in air.
On MGBs and A series Minis etc, the problem is two parts, usually the wrong fuel quality (Octane too low for the design of the head) and a build up of carbon residue that can glow red hot and act like a spark plug. It is a bit erratic as there is no control over the firing of the mixture so it tends to be a very rough run-on when switching off.
The MGOC sell an anti-run-on valve which essentially is linked to the ignition and floods the inlet manifold with air to drastically reduce the vacuum when you switch off and prevents the run-on. Modern unleaded fuels do tend to burn a bit hotter than the older lead based fuels so it is an issue, in part of the modern fuel.
Some suggest running the car, as I have found also on my BGT, on Shell V-Power for a few tankfuls to help clean up some of the deposits in the head.
However, ensure the timing is set for the 95 Octane fuel you are most likely using. The book figure is a guide only so it may mean having to fiddle a little on the road and listen for pinking. It might also help to increase the spark plug gap by a few thou, too.
If none of the above are of much interest, then the simple cure is to let the engine idle for about 30 secs to cool down a bit before switching off. If this doesn't help then put the clutch in as you switch off as this can add load to the engine and slow the idle speed down to hopefully prevent run-on (and yes it might help to set the idle speed down a bit on the carb). Lastly, if all else fails, then stick the clutch in, select 4th or in your case, probably 5th gear, and make sure the handbrake is on. As you switch off and the run-on begins, let the clutch up slowly to stall the run-on.
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