I am planning on machining a new Cylinder Head for my son's Pulling Garden Tractor over the winter. It is a single cylinder Kohler Flathead engine. Any thoughts on what is the best location for the Spark Plug? I know some people say it needs to be directly over the Exhaust Valve, some say centered on the piston, any thoughts?

Put in both.... and fire it with a Harley dual coil.

It will wake things up, but you will probably have to back the timing down a little.

On the typical Ford flathead engine, the spark plug valve is more centered between the valves, just off the edge of the piston.  This is considered a turbulent area.

Ford flatheads overheat if advanced more than 4 degrees static + 10 degrees vacuum advance; lots of advance is not needed.  I believe other flatheads are similar.  I think you are drawing inferences about the need for advance based on the shape of the combustion chamber that may appear to be rational but in practice are not.  

There is really only one way to get high compression in a flathead (in an existing engine);  larger bore or longer stroke or both -- increase cylinder volume.  To get flows in and out of the cylinder requires a certain amount of area above the deck (distance between the deck and cylinder head), but increasing the flow area lowers compression.

As far as Andy Granatelli; there were many true innovators in the flathead world "back in the day", I don't think I'd call him one of them.  Barney Navarro just died this week, check out his work.  

Relieving is of questionable benefit on all but the full-on race engines.  Similar results can be achieved thru shaping of the head surface, which is one of Navarro's tricks.  Relieving necessarily stops short of the cylinder walls, the head surface doesn't.  And relieving decreases the compression ratio... Wonder why you don't see any modern flatheads?!

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