I've got thermo fans on my blown Torana and it never gets hot, my 308 Sunbird is set up similar and has no probs. But I had a VN V8 wagon a few years ago and pulled off the factory fan and fitted a thermo and it got hot as soon as it got near traffic, so I had to refit the factory one.
Problem solved!

I'm actually having trouble with my VG Valiant wagon (245 Hemi) getting hot when it sits on 100km/h for any length of time. It's totally original, so I don't want to fit a thermo, but I want to drive it on the highway too.