Octane is just a number. It varies by part of the country, brand, and in some cases, street address of the filling station. Very, very general guidelines when using iron open chamber (Mopar type) stock heads): 87-89 octane can take about 130-140psi, 91-93 (hi test) can take about 160-180psi of cylinder pressure. Unfortunately, cylinder pressure has very little to do with static compression ratio. And that's what everyone tries to use for comparison. You cant compare two engines, even when built identically, using local fuel from two different sources. Even if the are the same published octane rating. Use a different length rod, smaller modern chambers, make use of quench effects, run tight clearances to keep oil out of the chambers, run coatings, run aluminum heads, or run modern cam lobes and centerlines, and you can raise the threshold of detonation, and in turn run more static ratio and get more power from the lower octane fuels. The simplest way to see what you may have trouble with, is to use the many online calculators to see what you have for dynamic compression ratio. That takes into account the rod length, and the cam timing events to give a realistic value for your engine. This is assuming you are doing the machining right enough to have values that are accurate. In "dynamic compression" speak anything near 7:1 is sluggish and will lack low end. anything around 8-8.1:1 is snappy, but may require at least the 91 octane 10% ethanol stuff I have around me. above 8.2:1 you will need high test, and you should be making use of parts designed with pump gas in mind. A typical 340, lets say with TRW "10.5"1 forged pistons, and the X heads and a modern cam, like the XE268, should end up about 9.3:1 static, and 8.25:1 dynamic. With no parts to work against detonation, this engine may need to have the timing curve carefully setup, and may need pump premium all the time. While the 505 i recently did, has dished pistons and tight quench, closed chamber aluminum heads, and a long rod, plus a cam that bleeds off pressure. And it has 10.6:1 static, and 8.2:1 dynamic. It runs great on high test, but should be able to run 89 once it's fully tuned. The difference in power from 9.3 to 10.6 is about 8% accross the board. That's just shy of 30+ hp on a typical 360hp engine. And it means any fuel, any where, can be bought and safely run.
