Re: 1960 5.5 lower unit removal??
You will need to remove the powerhead to get the lower unit down. You remove about 7 screws/bolts below the pan that holds the powerhead to the motor. Once the powerhead is off, you can remove 2 small hex nuts from the shift lever that allows you now to pull down the lower unit (provided you have removed the 4 bolts around the cavitation plate).
I would think your motor would have a thermostat. My 1961 5.5Hp Johnson does. It is in a little triangular piece just above the cylinder head, held on with 3 slotted screws.