91 Octane in a 340?

If it's a stock engine compression wise, it's not a true 10.5. None of them were what they were advertised as. They all blueprint lower than advertised, because of taller than spec deck heights and larger than spec chambers. Its true ratio is probably down in the 9s so I wouldn't hesitate to run 91. If it rattles, you can always go back to the 94 but I doubt it will.