[Blackeagle]

If the tops of the fins contact the under side of the top I see no advantage to holes over a single slit. They will only increase friction = increased head loss, for no gain in cooling. I also like the idea of a 5/8" barb, and drill it out to boot. Anything to maintain max flow until inside the block where the head loss can be traded for increased cooling. You could even insert a bit of copper 1/2' tube (or 1/2" joiner tube/fitting the true OD is just under 3/4" with those but this would only help if running over sized lines), then sodder in place. Disadvantage with that is they don't come out real easy for testing changes, but another possible way to go.

I do very much like the idea of a slightly inlarged opening for the center 3 channels. This could be done with a very small roto saw cutter. But I can see a couple of concerns in doing this.

1) you don't want to decrease the flow in the outer channels by much.

2) I'd start out with only about a 20% increase per inner channel to start with. You can open it later if you wish as you are testing.

3) A narrowing of the slit overall will be needed if a very large increase in the center slits is done. This of course to maintain velocity.

edit: I see Ben posted ahead of me on the copper tubing idea. I really like the idea/advantage of higher flow gained useing copper tube as inlets/outlets to blocks as opposed to barbs. Keeps head loss at a mimimum where it does nothing to improve cooling.

edit (2) for better clarity on the sizing of copper tubing to use.