I wouldn’t do the towel thing or use any other thing to reach into the fan while running - just asking for trouble there. Think about what a ‘good’ result would be - the towel ripping out of your hand and... If you’re somehow looking to confirm, just have someone shut it down at temperature and immediately try to turn the fan while holding the pulley (after it comes to a stop). The clutch won’t cool down in half a second, just like it won’t heat up in half a second. Oh, and you might be able to turn those derale blades around and turn them into pushers... then mount them on the front side of the rad. My 451 is probably in the low 500hp range and the (very nice) custom Mancini radiator with 5 blade fan and Hayden clutch struggles in idle / high heat / high humidity situations. The 2 pusher fans do the trick and drop her right down to the stat temperature in seconds.