Run on is caused by too much fuel in the chamber after shut down.
Big cams require lots of timing. I would use at least 20 degrees at idle.
In most cases this is what happens: Low timing brings down the idle speed, so the idle speed screw is used to increase the rpms, this will put the throttle blades past the idle circuit and into the main circuit which causes a very rich idle. Making it run on after the engine is shut down.
Advancing the timing and lowering the idle speed screw will cure run on.
If the carbs aren't letting enough air in at idle speed to keep the throttle blades in the idle circuit then you will need to drill a few small holes in the throttle blades or crack the front and rear carb open a bit.