The 91 should run just fine.
The calculation for octane today is different from back then. Basically your 390 is a high compression engine, and requires a higher octane to prevent pre-ignition, or knocking. It'll run on regular, but you'll hear something akin to marbles when you step on the gas. Higher octane actually prevents the fuel from igniting before the spark plug tell it to. Today's 91 octane is roughly equivalent to yesterday's "hi-test" of 100 octane.
There are two other notable differences between today's gas and yesterday's. First, the lead. This is grossly overrated and should not scare you at all. How soon we forget -- Amoco used to ADVERTISE unleaded gasoline back in the day! Now, that's all you get. If you use unleaded gas in an older motor, it is possible for the valves to wear out faster. If you do a lot of driving in the mountains, or pull a trailer, basically driving in such a way to be hard on the engine, you could experience valve wear over time. If you take your car on a Sunday drive to the beach, or to a Saturday night car cruise, it makes no difference whatsoever. If you rebuild your motor, they will generally install new hardened valve seats -- and then you can pull trailers up the mountains to your heart's content. It's just not a big deal.
The other difference is more noteworthy. Today's gas is pretty-much gasohol. It contains ethanol. Nothing wrong with that either, except your car wasn't made for it. Because of the ethanol content, it is wise to replace any rubber fuel lines with new composition. Heck -- that rubber is over 50 years old! Replace them anyway! The other side effect can be a nuisance. Ethanol gas, even high octane, has a lower boiling point. After driving around on a hot day, pull your car into the garage, shut off the motor, and a half-hour later your garage will stink to high heaven. The gas in the
carburetor bowl will boil and evaporate. You might also find your car harder to start, and easier to stall, when hot. Example, come out of the store or restaurant, get in your car, and it'll stall three time trying to get out of your parking space. Once you've driven a block or so it's fine. Yes, that's the hot gas. Spacer plates under the
carb can help.
Aside from knowing about these things, put that 91 in your car and enjoy the ride.
BTW, personally I use 89 and the car runs great. A couple of times I've accidentally used 87, and the car is still okay but I'll indeed get a small knock if I step on it. But -- my motor has blow-by, and the compression is a bit lower. Once I fix this I may need to go to the 91.
