Where are the timing marks located on a Chevy van g20 57l?

If this is a 305/350/400 small block Take the lead from the coil to the distributor "OFF" to prevent the engine from firing. This will take two people or a remote starter. Turn the engine over until the mark on the tortional damper lines up with the center or O mark on the timing tab on the front of the engine fastened to the crankcase front cover and the engine is in the number one firing position. This may be determined by placing fingers on the nuber one valve as the mark on the damper comes near the O mark on the crankshaft front cover.If the valves are not moving the engine is in the number one firing position.If the valves move as the mark comes up to the timing tab the engine is in number 6 position and should be rotated one more complete turn to bring it to the number one firing position. Also take the cover off of the distributor to determine that the rotor is positioned properly pointing to the number one spark plug then follow the firing order for that engine should be 1,8,4,3,6,5,7,2 Wayne