I asked the initial questions about the correct thermostat and whether I should replace a working water pump prior to having new a/c installed on an 89MJ that never had a/c previously. I had a mechanic install a/c without replacing the water pump or thermostat. Not long after that, leaks and rattles were followed by replacing the water pump and thermostat with a 195 'stat. I had the non-leaking heater core replaced with the a/c install because it is a big job getting to it. The heater control valve had never been removed previously which is a good thing. Otherwise, the heat would be on in the same plenum box as the a/c core. Winter came. 195 thermostat in place; good heat expected, right? Nope. Luke warm heat. We get winter but down in the 20s F is cold here. A next step might be to replace the heater control valve with the thought that either because of age or poor original design, water flow may be restricted. For me, optimal a/c is more important than optimal heat. I will probably leave well enough alone.