Not sure where you get an extra 1.5" in radius from? The larger rear will add 0.5" to the radius and I can't see the tyre itself adding a huge amount, offroad tyre sidewalls tend to be a similar size.
In your position, I'd remove the seat & throw a ratchet strap over the bike & attach to the swingarm. Protect anything you don't want marked with rags. Get someone to sit on the bike to compress the suspension & ratchet the bike down until the shock's fully compressed before measuring clearance. Ensure that the 17" tyre on the bike is not worn as this will throw everything out by the depth of the tread. If you've got at least 1", you should be okay. Perhaps get the circumference or diameter measurement from a (new) 18" tyre as a comparison?
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