A 275 horsepower 327 SBC (the base engine) was rated at 10.25:1 compression ratio when new. The "when new" part is important because odds are it isn't any more. Wear in the rings and piston skirt allows the piston to cock in the bore which keeps the worn rings from sealing as well as they did when new. That wear also removed metal from the cylinder wall as your cast iron rings are just as hard as the cast iron cylinders; so each wear away at about the same rate. As the cylinder grows the spring tension in the rings relaxes allowing even more cylinder pressure to escape into the crank case. So though your flat top pistons have picked up about 0.005" of lead deposits on the top of the cylinder increasing the static compression, the wear has lowered your static compression rate even more.
Back in 1967 regular leaded gas had an octane rating of 94 octane for the cheap stuff. That is a full two point above what most gas stations pump today though where I live 93 octane is the norm at the premium pump. In Georgia and LA (lower Alabama) many of the gas pumps have one that is locked except on Friday and Saturday night. That is the 104 Octane race gas pump that they have to have to fill up all of the cars going to race at the short oval, the long oval, the high banked, and the dirt track that is in every open area around the Southeastern part of the US. You still can not put it in your car; because there is no road tax paid on the gas sold (of course if you pay the road tax, or about the equivalent of 12 16 oz cans sold in the big blue box with a familiar beer logo on it) to the clerk or the clerk's boy friend who is hanging around because he has no beer money and he hasn't anything else to do, he will pump it for you).
Lead additives are still available on the shelf at auto-parts stores; and it will raise the octane of the gasoline as well as lubricate the upper cylinder. Tetraetyl-lead is made today in Germany instead of Louisiana, but it is imported to this country and sold in 30 gallon drums at any chemical supply house. It is highly carcinogenic and poisonous which is why it was removed from the gasoline supply in 1978 (three years later than intended). It will raise your gas to any octane you could desire, when mixed in the correct ratio with the fuel.
Back in 1967 all gas was received in your home town as regular and lead was blended into the stock supply at the distribution point to make the other two octane ratings you found at the pump (mid range and high test). You shouldn't be adding octane booster (which is just Toluene or lacquer thinner) if you are also adding tetraetyl-lead. As I said Toluene is sold as octane booster but there is so little for such a high price as to be a waste of money. Add a one gallon can of lacquer thinner instead to 20 gallons of 92 octane gas and it will raise the contents of the tank to about 95 octane, which is more than enough. Unfortunately adding more Toluene won't raise it much more than that; as it isn't linear as to how it works. To get to 104 lead free gas they use about half gas and the other half consists of other exotic aromatic compounds to raise the octane, including Toluene. As you might expect with a tankful of aromatic chemicals they will quickly start to evaporate out of your tank so do not mix up more than you can burn, as after a few days all you have left is the 93 octane gas you started with.