On those engines that have no hole tapped for the sender unit, the boss is still there and can be drilled and tapped. I did this on a 2005 4.0 engine block - the hole is for a 1/8" NPT thread. If you decide to drill and tap, be careful not to turn the tap too far into the hole while threading because you'll end up with a loose fit. This is tapered pipe thread, designed to give you a real tight fit when done right. When you drill the hole, it'll open up into the coolant cavity below the boss - any bits of metal that fall into this cavity that you can't retrieve with a magnet will disintegrate in the coolant eventually without harming the engine.