Drilling oil galleries and oil return holes?

Not knowing the stresses in the block at the lifter bores, and not seeing any big benefit, I don't think I would ever bother with those holes by the lifters. The oil quantity there has to be pretty small.

The enlarging the passages from the mains to the lifter gallery I DID do. It will put a little more pressure to each bearing, and thus keep the flow a bit better to the mains and rods. The front bearing holes need the most attention, as that is the crossover path for the oil from the passenger side gallery to the driver's side. Also, the passages out the oil pump and to the filter and back up to the passenger side lifter gallery.

Just drill those long holes at a steady RPM, not too high and not low to avoid chatter on the bit, and not too much pressure. Pull out often to get the chips out. Read up on drilling cast iron.