You can get those speed nuts at NAPA. You will have to match them. You might want to measure what the speed nut will be and get off of Amazon.
Been a whole since I’ve done a 6,0 cooler. I do think I remember they had larger lines and I adapted with Gates brass 1/4” NTP fittings.
They will allow you to use 5/16 or 3/8 fittings.
For example. Cut the 5/16 line coming out of radiator. Get 5/16 compression to 1/4npt Union to 3/8 barbed to 1/4 NPT. Use yellow Teflon tape on threads. White is not compatible whith petroleum products. You can get all of these items on Amazon.