Transmission cooling line fitting size help

Hello, finishing the plumbing on a transmission cooler line. I am stuck - I can not find the fitting size/type for the hard line that runs from the transmission to the radiator

My goal is to put the 3/8' hose barb on the hard line and run it to the cooler. Should have been simple

So far -6an (-4 fits into the radiator and does a -5 exist?) and 1/4" npt do not work-to large

I attached a picture of the fittings - I am looking to replace the brass