Sxr bypass fitting

I recently bought a aluminum ADA bypass fitting in 3/8, to replace the plastic and rubber one. Of course Kawasaki made the hole larger than it needs to be. I got the 3/8 fitting to sit correctly with some washers.

On to the next problem, apparently the water line is 1/4 or metric hard to tell. Can I replace the brass fittings coming from the head to 3/8 hose? Or will that affect the cooling of the motor. Engine is bone stock. I could use a 1/4 to 3/8 barb fitting.
 
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