I'm assuming because the m10 is a NA low compression engine regular would be the right gas, but I thought I'd ask anyway.

Any opinions?

Regular is fine. If you have a knock problem you can unlock and turn the distributor, too.

I run 85 oct with cranked ignition timing, but I am at altitude with a centrifugal-only dizzy.

Manual will tell you at least middle 89 or so, mine has never liked regular, but this is California gas!!

Octane is the resistance of fuel to burn. With that being said, most stock cars cant create enough heat to burn efficently 89+ octance. Ever been behind someones car that smelt like rotten eggs? Well its because their using the wrong fuel. The gas is being have burnt and leaves the engine and sticks inside their cat and muffler which ends up smelling like rotten eggs over a period of time. When your turbod you use higher octance gas because its more resistant to burn and pre-ignite(detonate.) If youd use lower octane on a turbod car it would burn up(detonate) before it ever reaches full capacity.

You might also try running mid-grade gas if you have a knock to settle out your detonation during ignition.