I got a 195 F thermostat for my 65 Dart and 64 Valiant for better mileage, since I have the hoses off currently. The Dart had a 180 F, not sure about the Valiant. I have read it makes a noticeable improvement. I think all late 70's models used 195 F for less emissions, though the 195 F came up as the standard for my cars on several internet sites. Many people install a 180 F or even 160 F thermostat due to overheating problems, but not sure that helps. I expect when they overheat, a 195 F thermostat is already full open anyway. SB (and probably BB) use the larger size (~2.5"D) and the slant six uses the smaller (~2.25"D), though 80's SB may use the smaller one since one store brought that out when I asked for a Diplomat 318 one.