I have a Datsun Pulsar Thermo switch on mine. They came out with a A14 engine here in South Africa but the engine was mounted to pull the front wheels with a front wheel drive gearbox. The stats of the switch says that it kicks on at 85C and off again at 81C.
So I would say this is the optimum temp for a A series engine.

Put in a thermostat as soon as possible. Running a engine too cold will cause premature wear in the engine because it is not running at it's designed temp.
Almost like a runner that does'nt stretch and warm up his muscles before a competition.He will pull a muscle.