allow me to drop an asteroid into the pond
don't design the block with the aim to make life easy for the coolant to get through it.... that is what bigass pumps are for!
design the block to maximize the energy stored in the velocity of the coolant, by converting it in as much turbulence as possible. Turbulence is the "scrubbing agent" you need to get the hot layer of coolant hugging the surface of the block, to let go.
you will find this philosophy to have a double edged effect, in the form of....... the more imaginative the turbulating inventions are you come up with, the more surface area you are going to end up with... Now that is a double positive. Believe me! not too many of those around these days...