In general, as long as there's oil weeping from the holes in the rockers you have sufficient idle speed .
Slow idle uses far less ful and gives far better engine braking if you're actually driving your old truck .
If not, up to 1,000 RPM's but I prefer 450~500 RPM unless the engine is worn out enough to foul the spark plugs ~ then you have to tune it more sharply and will be wanting at least 700RPM....