I'm not materials expert, but wouldn't it make a lot more sense to mill these out of aluminum so they expand and contract at a similar rate to the coolant tubes you cut....
Yeah, I think you're onto something there. Maybe what you're suggesting, but making the whole thing out of aluminum would probably be less leak-prone, more durable, and give you more cooling to boot. Would probably cost significantly more, but would be the best solution short of welding.