Hi,
Yes. Don't block up the underfloor vents as they will build up moisture levels under the floor of your house. This will rot out the timber bearers and joists and also wet timber is an invitation for termites.
If you are in a bushfire area, your vents can be made of either: heavy duty aluminium, stainless steel or bronze mesh sealed to a fire rated wall such as (13mm fire rated + wet area plaster covered over with 6mm fibro cement).
Have you thought about packing insulation - for example fibre glass R3.5 batts - under the floor of your house? I've done that here and the timber floors are toasty warm (or at least not cold) and it gets cold up in the mountains north of Melbourne!
Nothing you can do about concrete floors. They are always hot in summer and cold in winter because they settle at the average ground temperature which in Perth can vary quite a bit.
Given the age of your house (1970's), it would be a safe bet that you have no insulation in the walls? I could be wrong, but at that time no insulation was put in. It has been only recently that building surveyors requested and checked this as part of a house construction requirement.
You can check for insulation in the walls by removing a light switch or power point (be very careful with this and remember to switch off the power) and have a look up into the wall cavity.
If you have weatherboards, it is a lot of work, but pretty cheap to do, but will take a lot of time, pulling the weatherboards off and packing insulation into the walls. They now sell R2.0 fibre glass batts for 90mm wall cavities and they are worth their weight in gold. Whilst you are at this put in a moisture barrier lining too as there may not be one. Plus with the exposed wall, it would be an opportune time to rewire and replumb the house.
If you have a little bit more money to blow: remove the internal plaster on the external walls and build another timber wall adjacent to the current external timber wall - so that you would then have a double stud external walls (make sure you match the timber studs). Into this you could pack R3.5 batts. This is the design I have here and it never gets below 12 degrees inside (without heating) over winter despite being sometimes less than zero outside. Over summer it never gets above 26 degrees inside despite being over 40 degrees outside.
You could also add a second layer of insulation in the roof. If you have a tiled roof, it is virtually open to the outside world and back in the 1970's they didn't install moisture barriers between the tiles and timber either.
Good luck!
Chris