The reason it is drilled larger at the saddle is there is an intersect point from the true main galley feed for the mains, to the cam bearing feed. The holes to the cam bearings don't need to be as large as they are, so there is absolutely no reason to open them up. In fact, making them larger would only tempt fate with as narrow as the webbing gets between the main bulkhead and the cam. Think of it this way... any oil going through those drillings up to the cam is robbed from the crank and rods. The cam needs minimal oil supply on it's journals.
There is a simple modification you can do to supply the heads with oil without starving the mains/rods. You turn the 4th cam bearing to block the holes that lead through the valley into the cylinder heads. Then drill through the valley drilling into the main lifter galleys for an oil supply.