LA heads on magnum block

All I can say is that is a long drill distance to keep straight; wandering off would be my concern. If you get off even a bit, the holes in the cam bearings will not line up with the rocker oiling hole and also with the cam oiling hole from the mains. Even worse, you could bust out of the web before you got to the cam bearing.

If I was to do this, I would drill a small distance into the block below where the head oiling passage enters the block, and then drill an intersecting passage from the valley. Then tap this hole and plumb with oil control restrictions of some sort to tapped holes drilled in the passenger side oil gallery.