Octane requirement question

I've got a 300 hp crate 360 magnum engine in my 91 power ram 150, it was purchased brand new by my father in 2003 and I acquired it in 2010, it's a low mileage engine, it had 18,500 miles on it when I got it, and I have barely put 5000 miles on it.
My father always ran 91 octane in it and said it had to have 91 ran in it, we live in Idaho and we are 4000 feet above sea level.
This engine is advertised with a compression ratio of 9:0.1 and I checked the cranking compression with the engine fully warmed up and the carburetor opened up all the way and every cylinder read 150 psi on the dot.
I've been running ethanol free 91 octane fuel in it ever since I got it, but I noticed the other day the only gas station that sells eathanol free premium jacked the price of it up to where it is 22 cents a gallon more then the standard 91 octane with 10 percent eathanol.
Fuel prices where I live and their octane rating is 85 octane with 10 percent ethanol at 3.04 a gallon, 87 octane with 10 percent ethanol at 3.14 a gallon, and 91 octane with 10 percent ethanol at 3.24 a gallon, and of course ethanol free 91 octane at 3.47 a gallon.
Oh and there is a station that sells 87 octane with only 5 percent ethanol for 3.14 a gallon as well.
I'm just wondering can I run 87 octane with 10 percent eathanol in this engine and be ok? I've been told when you live higher in elevation the octane requirement is lower because the air is thinner, so I was thinking since I'm at 4000 feet above sea level, and the compression ratio is 9:0.1 that I should be safe running 87 octane, I don't know much about cranking compression and how that helps determine the octane you should run, but all 8 cylinders read 150 psi on cranking compression.
I'd just like to save 33 cents per gallon at the pump if I can, because that's quite a bit. Specially when your filling up a 30 gallon fuel tank. That's darn near saving 10 bucks right there.
I know timing plays a part in this and my initial timing is 16 degrees and my total timing is 34 degrees and I have another 14 degrees of timing added at idle from the vacuum advance.
And my timing is all in at 3000 rpm.
What are your thoughts? Should I be safe running 87 octane fuel?