Hole in casting... Fill or not?

According to the manufacturer, JB Weld is "completely resistant to water, gasoline, and just about every other petroleum or automotive chemical."

The lifter waist band will move up to that hole area and probably lose some oil. It may well be a leak to the oil system (kinda like the hydraulic roller problem in a 340 block with the 340 lifter bore chamfers). Any leak will be much more serious if on the passenger side. So I'd plug it.