GM spec: 130 psi minimum (higher was OK).
http://chevy.oldcarmanualproject.com/shop/1954truck/54ctsm0617.html
http://chevy.oldcarmanualproject.com/shop/1961ctsms/61ctsms0701.html

I think another spec for all years was no more that 5-10 psi difference between/among the 6 cylinders "for satisfactory performance".