temperature

i got a 327 in my 86 chevy truck and i am running the same temperature guage that was in my 305. The 327 ran max at 155. My mechenic said that i might not be running a thermostat, so i bought a 160F one and took my goose neck off. Apperently i was running a working 180F thermostat, so i took it out and put my 160 in. Now it runs at 115 to 120 max. I dont know if the sender unit is working right or whats going on. any sugjestions? Also the water flow is not deverted.