One propane torch doesn't do much. With a 3 blade prop, I'd think 3 would be a minimum - one between each blade. You will NOT hurt the prop with air/propane. I've pulled props off shafts larger than 2' diameter with ice forming in the river, using steam and strip heaters (and a hydraulic puller). If you're worried about the temps of the metal, use in IR (non-contact) thermometer.
Oh yeah, don't remove the nut - just loosen it a turn or so. When that prop comes loose, it will do so with a bang, and you don't want to be behind it.