I'd try the Derale if you have the room and a good alternator that can handle the load. Make sure to use a good relay. Can always fall back to the clutch fan if you have room. Derale & Spal are great fans typically.
Yep, the $5.00 meat thermometer is the easiest way to check. Get it warmed up, make sure the stat has opened, then pop the cap & test. Many times the stock gauge is just off.