I have run the grooved heads with both .040" quench distance and .070". The reason I recommend .070" is because the engine is more resistant to detonation with the wider gap.

Let's discuss the sequence of events as the piston approaches TDC on the compression stroke. As the flat surface of the piston top approaches the quench area of the cylinder head the mixture in that area is forced to flow into the combustion chamber cavity. If the quench area of the head has been modified with a groove the fluid flow into the combustion chamber cavity is altered. When the spark plug ignites the turbulent mixture created by the altered fluid flow from the squish area, accelerated combustion races towards the squish area.

Now think about the shape of the cavity formed by the piston top, the quench area of the cylinder head and the groove it self. If the flame reaches this area with the piston at or near TDC and the quench distance is too tight, the quench area will not support combustion. The end result is unburned fuel in the quench cavity.

Some people choose to run the quench clearance so tight that the piston almost touches the head. Experience tells me that if I set the quench distance to .030" at engine assembly the pistons will come very close (or touch) the head at RPM. If one piston happens to be closer than the others, disaster will ruin the day.

I find that with the grooves the clearance can be opened to .070" and performance will improve in spite of the fact the compression is lowered.

I recommend milling the heads to obtain the desired compression and set the quench distance with the proper thickness gasket.