91 Octane in a 340?

More like 12psi:
so if you build to 180 psi with alloy heads, and make no changes, then the Wallace calculator predicts 192psi at sealevel. You can run ~87gas up in Edmonton, and 91 down at the coast, or maybe 89 on the prairies which are around 1000ft or less. Same engine; no problem.
But
if you build to 165psi with iron heads, at Edmonton, and then drive down to sealevel, the Wallace predicts a rise to ~177, which it gonna rattle the engine pretty hard every time that you lay into it.
Here's the kicker;
when you get to 5000ft crossing the Rockies, Your Edmonton pressure of 165psi, will have dropped to 150 and your 340 is getting a lil lazy.
Highwood pass is over 7000ft, so now yur down to 140psi. You might need a supercharger....... lol.

A change in elevation requires a change in mainjets (as does temperature), to the tune of about one size per 2000 ft, IIRC.
Therefore if you plan on traveling vertically, you really need to consider the size of your engine, and various pressure changes that it will go thru
Your 340 will get you over Highwood pass, but 7000ft requires 2.5 jet changes on the way up and 3.5 on the way down to the beach, lol., and 140psi is getting sluggish.
This elevation-business requires a careful co-ordination of parts, including the size of the engine.
if you plan on spending any significant time at sealevel, on the coast, then, IMO, you gotta set your pressure close to the max for the lowest expected elevation, and you get what you get in Edmonton, else install EFI, or, leave the car at home. This is the reality of it.
BTW
if yur thinking the 340 is the bees knees, for this kindof application, I cannot agree. Well, it could be..... with a stroker crank in it...... but then...... it wouldn't actually be a 340 anymore........ altho, only you will know it.
Well and the guy you just blew off at 85 mph! lol