Radiator cap

My 67 273 did the same thing if I filled the system to the cap. Somewhat embarrassing to roll in at the cruise-in and dump a pint. One solution is leave the fill level low, barely cover the tubes like the factory prescribed back then. Then it dumps a little air if anything, and the air adds to the rusting and all that is why they added a reservoir. That's why I added a reservoir to my 67 too. I found the translucent plastic reservoir quite beneficial in that I see the cooling system condition at a glance, while pulling oil dipstick, whatever.