[Dutchdivco]

There are hardened exhaust seats available for 1965 engines, as unleaded was already prevalent by then.And I believe you have it backwards; gasoline was 75 octane gas 'back then'. Higher octane means LESS 'bang for the buck'.i.e. octane effects, in plain language the flamability or ignitability of fuel.Higher octane means less ignitable.When they started putting 'extra' gas into the cylinders,in order to make sure there would be enough unburned gas in the exhaust to make the catalyctic converter work, they had too much pre-ignition. Even with knock sensors, so they had the gasoline co.'s make the gas with a higher octane, making it, in effect less flamable.They called the highest octane gas "premium", implying that it was the 'best'.In addition, for a long time there was the belief that certain engines, like Cadillacs, needed to run on "premium". I don't know whether this was true at 1 time, or whether this was part of the snow job to convice us that higher octane was somehow better.My understanding is that currently, there are no stock cars which "Have" to run on premium gas.Cars which have had work done on them to raise the compression, yes.Anyway, the 'old gas" was 75 octane, and it really did have more bang for the buck; more volatile, more easily vaporised, more flamable.
One other point; economy includes all the expences of operating your vehicle. Both insurance co.'s and state registration (DMV) go by the kelly blue book, in order to calculate the charges. And the Kelly blue book doesn't take into account "collectible' status in its values. The new price starts out high, and then gradually declines.In Az., a 1965 vehicle is charged the mandated minimum for registration, $15.00/year.Insurance, at least mandatory liability, is similarly very low.More $ in my pocket, or to be used for financing my mod projects! Jim