Gas options

The 93 octane recommend in 1973 is probably the Research Octane scale.
Today, the octane rating displayed on the pump is the average of Research Octane and Motor Octane. Generally, Motor Octane is around ten points lower than Research Octane, so lower that 93 by 5 to come up with 88.
Try the 87, make sure your timing is correct, and maybe use a combustion chamber cleaner like Gumout Regane of Chevron Techron.