91 Octane in a 340?

Cylinder pressure is a much better indicator as to if you need to run a hi-octane or not. This is what the engine actually sees versus an abstract number that has very little usefulness in reality.

The Compression ratio formula reads;
(CV +cv)/ cv , where CV is the swept volume, and and cv is the total chamber volume. all are in EITHER cc or cubic inches, Generally ccs are used, do not mix units.
So a stock340 has a swept volume of 696.364cc
And the typical total-chamber volume, with an .039 headgasket, is gonna come in at around 81cc, so that makes her a 9.6Scr engine.

In Edmonton, at some 2130ft elevation, you can take a 10.5Scr 340engine, which in reality is likely closer to 9.6Scr as shown above, , and just by changing when the intake valve closes, vary the pressure from say 152psi, down to 124psi, or even less.
>With open chamber iron heads;
At 140psi or lower, you should be able to run 87E10 in it no problem.
at 150psi, probably still on 87, but the low-rpm/hi load may say otherwise at your early all-in timing setting.
at 155psi, for sure needs 89 (or better)
At 160psi, 91 is getting iffy. The power-timing may have to be delayed or reduced. Your engine is not likely gonna like the current 33* @2500, unless the stall speed is over that, and the chamber temperature is kept in check.
at 165psi, everything has to be just right to get away with 91, and more likely is that the timing is gonna take a hit.
<But notice that it's still a 9.6Scr engine.>

In Edmonton, at 2130ft elevation, your stock Hi compression 340 longblock, with a 268Plus roller cam, is gonna be down on pressure, so
I very much agree with those who recommend to do a compression test.
Pressure makes heat, makes power...... so long as full-timing can be used, which on an iron-headed 340 is usually 36>38 degrees, after 3600rpm.

I wonder if you are fixated a lil too much on the power numbers.
On the street, using just two gears, 250 hp is plenty to have fun with, because you can adjust the output with gears and stall. However 2130ft elevation is already about a 10 psi handicap from sealevel, which you can definitely feel.
On the street, you are very seldom gonna see max-power in Second gear, unless you are running an automatic, and 3.91s or better, and don't mind speeding.
IMO, with a noisy exhaust system, it is as good as impossible to hear detonation. But you can devise a test for it, and if severe enough you will for sure feel the hammering going on under the hood.

As for the test:
The test is just a time trial over a measured, distance or speed range. But you gotta load the engine up to prevent tire spin, and at some rpm after the mechanical timing is all-in, and your method has to be repeatable, To that end, if an automatic trans is used, use Second Gear, all by it self. I use the speedOmeter, from whatever speed 3500 in Second gear is, to at whatever rpm the cam peaks at, plus 200 or so.
Say you have 3.73s; 3500 will be ~45 mph, and 5600 will be ~75 mph. So make the test from 45 to 75.
For the following tests,
>if at any time, the car slows down, abort the testing. A slow-down can only mean ONE thing, namely, that the engine lost power. And that would most likely be, but does not have to be, because of detonation. But in any case, we don't want to go slower.
> also, repeat all tests at least twice and average the results.

Set your power-timing to 33*@3500.
Get a helper with a stopwatch in the backseat looking over your shoulder, then motor up to 40mph in Second, then floor it. When the Speed-O needle crosses 45, the helper starts the timer and when the needle crosses 75, the helper stops the timer. You watch the stinking road, lol. This will be your baseline.
>Now reset your Power-timing to 31*@3500, and repeat the test. If we're losing power, it will take longer. This is to be expected at just 31*..
Next, reset your Power-timing to 35*@3500; and repeat the test . If the car is making more power the time will be less, good news, we can move forward.
One more time; reset the Power-Timing to 37* at 3500 , and repeat the test. If the time is longer, do not repeat the test for a second time. Go back to 35*, and call it done.
BUT
If the time is again less, repeat it a second time and average the results. If still quicker than it was at 35*, Then Stop the test, lets not push our luck, ......37* is your magic POWER-TIMING number, on this day, with whatever gas was in the tank, at this location.
Now;
I want you to compare the averaged times very carefully. because the times should be very short, and closely spaced, within a second or so from quickest to slowest not including the one at 31*..which is why you need a helper for consistency.
But the point is this;
compare the times at 33*, to the times at whatever best, that you got. Now ask yourself this:
Is it worth it to you, to run on the ragged edge of detonation, to get that portion of a second quicker? If you answer yes, then, IMO you need to reconfigure your 340, or you need a different engine, lol.

Ok then, now that your power-timing is figured, you can tune the timing from idle to 3500........ And then the Vcan, and ten the cruise-timing....... lol
If at any time you change mainjets or PVs, or Scr, or pressure, or your heads, etc, then you gotta revisit your timings, all of them, starting with the Power-Timing..........
Happy HotRodding.