I'd go with standard torque specs, here's a list,
http://www.dansmc.com/torque_chart.htm/ I can't tell from the pic if it's 6 or 8MM. The way to tighten (or loosen) on something like that is to use one wrench on one of the nuts, locked against the hub, then turn the one you're trying to tighten or loosen while it's held in place. I just did this on my Grandsons Toyota a couple of weeks ago, same design. It's not the smoothest operation, but it works.
The radiator is probably ok, the fins are no worry at all, that middle tube is the one thing I can see that might be a problem, but I think you'll be ok. Just keep an eye on it when it gets hot and under pressure.