[ubernoob51]

I run "premium" 91 octane here in California. Ethanol-free fuel is incredibly difficult to find here, other than expensive race gas or leaded aviation fuel.

BUT: I have flat-top pistons and a smaller-chamber 1.5 head. Even with a modified distributor with limited mechanical advance I definitely get pre-ignition with regular fuel.

When I had the head rebuilt I had 1.9 valves and hardened exhaust seats installed. From what I can tell it's standard practice amongst machine shops here.

I also heard that California's fuel is blended to produce lower emissions, which makes it burn hotter and makes installing bronze valve guides risky. The owner of the machine shop where I had my motor rebuilt said he had a lot of heads come in with galled valves/bronze guides back in the 90's, so since then he only allowed iron guides to be installed.

To answer your question:

For a stock '73, unleaded 87 is fine. Those engines had low-compression pistons, induction-hardened seats, and a much less aggressive ignition timing curve than the pre-71 engines.

On the other hand, a bone-stock 69 19S? I would hesitate to run it on anything other than avgas without modification. I think the distributors had ~60° of mechanical advance! They were definitely designed for high-octane leaded fuel that is unavailable for the most part today.