Argh, I lost my post.
Firstly for the sake of $15 I would replace the thermostat, or at least test it.
I don't agree that it is necessary for a 30 year old datto to run hot all the time, even in a Brisbane summer.
I've got jetcoated headers, non-vented bonnet and a non-shrouded fan. It doesn't run hot. Bonnet vents will be helping a lot more that ceramic coating the exhaust manifold.
Is it possible that you are running lean? Do you get a lot of overflow coming out through the radiator cap/drain?