As above....try richening the HSN a little bit. As for airleaks, ....if you have one, you have to richen the needles a lot, so even the factory spec settings may be still too lean. Typically the carb pinch bolt (what you tighten to hold the carb tight) is a cause for leaks. RTV is good to fix that (I've used regular with no noticeable problems, but the oxygen sensor stuff is the recommended RTV).

Long runs really build up heat. You have to tune a bit more rich for this type of running.

Is there lots of air getting to the cooling head? Some people cut more out of their bodies to help with this.

As far as temps go, I've accidently run one of my engines up to 400F once and over 300F several times and it was fine, but I like to keep the temps between 200 and 300F. It's nice to get temps around 240ish with a peak no higher than 270F as the vehicle runs out of fuel. The 290F probably won't hurt it, but you are getting to the point where the fuel (lubrication) may not be enough, or might boil off, etc.

With some engines, performance vs temp can be a trade-off. After-market cooling heads are "cool" and work well, but you could mask an overly lean condition if you arn't careful. A shorty cold plug, such as the OS A5 works great (reduces compression in an engine designed for a normal long plug, and retards the timing). An extra head shim can help, but you may have to re-break-in your engine a bit for the new cooler running depending on the shim thickness.

Inreasing heat as the tank fuel level drops can be reduced by shortening your pressure line as short as possible. Helps, but you are always going to be stuck with it too some extent. Hmmmm....I take it your pressure line is hooked up.....

All this aside, judging from your post, do as the other fellows suggest....richen the HSN and see what happens. As they said, if your HSN is too lean, the engine will get hot. But you may see a lot of smoke because the LSN is probably too rich and it takes a while for the engine to burn off the excess fuel during a high speed run. If the high-speed run was long enough, you'd probably see a reduction in smoke. Unless the LSN is very rich, you can tell this is happening....when you suddenly get off the throttle, the engine will race for a little bit and then settle down to a normal idle.

Some people say things like "if you over 240 your engine is toast", etc. Others say things like "Don't worry about the temps, just tune for best performance". I think the real truth is a bit of both. The exact temp isn't so important, but too hot is bad, and too cold is bad.

There...I've said enough that I've opened myself up for criticizm. That's fine. I'm not an expert, but I did own the most annoyingly over-heating engine known to man.....the Force 26!!!