Coolant spewing from my overflow hose

hey guys,
I have a new 3 core radiator on my 72 Demon.
Temp holds well when I’m driving about 180-190, as soon as I sit in traffic or turn my car off I get coolant coming out of my overflow hose.
I don’t have an overflow bottle.
What psi cap should I be running?
What do you guys think could be the problem?
Cheers