You didn't say what year your 235 is, but this page 1954 Chevrolet Truck Shop Manual from the 1954 Chevrolet Truck Shop Manual says 450-500 RPM.
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....