It happens to the best of us. You need to get your elbow calibrated to act as a torque wrench :naughty:.
Does the Autometer sender have the same dimensions as the engine coolant temp sensor on the water pump? A simple R&R would be ideal. If not, I'd still like to have temp info coming from the head/intake area (where the greatest T fluctuations occur) & not the block.
I'd leave the broken sender where it is; so long as it doesn't leak, and there are no loose edges to catch on anything (or anybody).
...And I wouldn't worry about metal shavings, better in the crankcase where they'll get caught by the oil filter, than up in the head. I guess if you wanted to be anal, strong magnets are available that you just slap on the oil pan.
Post back with where you put the new sensor.
g/l,
-W-

btw: the plug you saw on the block was probably the freeze plug, designed to pop out should the engine freeze from lack of antifreeze it lowers the odds of cracking the block.