stock slant six , which grade of gas should i use ?

hi , just picked up a stock '74 valiant with a 225 slant 6 , auto . electronic ignition . 144,000 miles . bought it from a dealer and they don't know what is in it gas wise . i was wondering what should i use ? regular 87 . plus 89 , or premium 93 ? i am on east coast in NJ . any recommendations are appreciated . obviously i would like to use the cheapest gas , but don't mind paying extra if its worth it . thanks