What actually controls oil flow to the shafts is the amount of time the holes in the cam line up with the holes in the block.
You can do it externally. I have.
I used #4 line from the oil pressure port. Used a T to feed both sides. I used a bulkhead fitting in both valve covers. Then I tapped into the rocker shaft at the regular feed spot on the shaft. That will give you full time oil to the rockers.
I am not familiar w/ small blocks. If they oil from the cam holes lining up, like previously stated, why couldn`t a .030 groove be cut in the appropriate cam journal ? Wish I had went that way on my big block.