Thanks for the info, Epoch!

I am living in Australia and it is almost winter here. In the morning, if I drive onto highway or freeway, it never gets to the normal temperature. It stays at about 1/4 towards 'C' on the temperature gauge. Heater is working fine. When I turn on the fan to level 1 and the heater lever is in the middle, the temp of the air coming out from the vent is about 35C.

I have the Helms workshop manual. It looks simple in the manual. Maybe like what you said, I should get someone who has done it before and I'll drain the coolant before pulling it apart.

By the way, what sort of temperature rating of the thermostat should I get? The recommendation in the Helms manual is 195F. Which temperature rating is better to the engine?