If your that concerned about a possible valve burning, you should investigate what the racers use. Light weight ti or somesort of heavy duty Incconnel valve for high heat. Of course your looking at a full set of special seats.
Like I said before, it really doesn't buy you much until you turn up the timing. The USA cars came 6 degrees retarded timing. Maybe you should look through the performance archives here and set yours up for the higher timing. Of course, the Euro's had higher compression which helps you get more hp out of that fuel/timing.
I think the use of sodium valves is not just due to heat or heavy duty, but is an engineering repair for a weak design. By weak, I mean the geometry of the exhaust port, stem length etc, there just is more too much heat. Common on AV aircooled engine as I'm assuming like their street brothers run hotter.
Diesel stems are 9mm I believe where a gaser is 7mm. If you choose to purse it, check lengths/sizes... then ream the head out to take a diesel valve guide. Be aware of total valve weight with retainer as that will effect the valve float point (RPM) of the engine.

Michael