Yup it is, my 340 is a true 10.5-1 and it runs fine on 91 octane. It runs even better with a race gas mix but most of the time I'm burning 91.
Last night i filled it with some gas that was probably around 90 octane and I drove it today and it was fine. Not as good as last night before the fill up but it runs fine on 90. Now I do have aluminum eddie heads and KB 243 pistons.

I did run it on 91 octane when it had factory hi compression pistons and iron heads also for many years.