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Normally it's better to have a tank that's longer and wider and not as high. The ocotpus moves over the floor of the tank. However, some do keep their octos successfully in higher tanks (I believe Carol used an octagonal tank for Ink). Since this is a 90 gallon, it would be more than large enough. So yes, you could use it as an octo tank.
I'm glad you're planning on 4 months for the cycling. You should be able to build up a nice supply of amphipods to get your octo off to a good start.
Actually, though, my last octopus (we couldn't ever quite place what species it was...there's a picture posted on here somewhere, though) liked to climb up high and hang out more up there than play in her lair. Of course, this was after getting out of the initial shyness stage. So there's something to say for aquariums with height.