Hi Marty - assuming yours is a stock '73, and not knowing where you live, I'd say
Any commercial OAT or ethylene glycol antifreeze, diluted according to your climate zone, should be fine. Prestone, etc.
Add coolant directly into radiator (fill to seat of neck). Then drive check again, and when cool top it off if needed. I'm pretty sure that your year car should have a 13 psi cap; if it’s old you might want to replace it.
Tom M.