Full disclosure I have no technical background, but understand that lead was the original octane booster as well as a way to cushion unhardened valve seats. I’m not sure when Chevrolet started using hardened valve seats…I want to say it wasn’t until the late 60’s. If that’s right, and your engine is a high-compression version AND has never been opened and hardened seats used, it MIGHT not be a bad thing to use a lead additive. And no more miles you put on it, the extra cost would be negligible. If it has had hardened seats added (like many by now probably have) and you don’t have a problem with knock or ping, I’d think you’d be fine just running a good non-ethanol 91 or so in it.
Off-hand, I can’t think of any harm MMO would do. But in my experience it’s as much a solvent as oil. I’ve heard it may have some small effect at reducing carbon deposits, but with modern fuels, modern oil and with prudent change intervals, I’m not sure of the benefit.
At only 300 miles a year, I’d keep the fuel stabilizer even with non-ethanol.
FWIW, I haven’t used anything in my '54 235 over the past 35+ years and maybe 35k miles since it’s rebuild (with hardened seats) except ethanol-free fuel when I can get it…which is 89 or better where I live. And at the factory timing settings, that’s plenty high octane for the old girl.