The governor is inside the injection pump case. I have an MD, not a 400, but I think they are similar. It sounds as though you may not have the friction control which is part of the starting control mechanism mounted on the side of the block adjusted properly. The speed control lever on these engines is tight and takes some effort to move because of that friction control. If not set tight enough, it could be letting the speed control go to max. If it is not that, the problem is going to be inside the injection pump. Is the engine running away, or just going to max high idle??
To adjust the gas starting valves, you have to remove the valve cover and adjust the clearance. Starting valve clearance for the MD is .060 to .080 inch(valves closed, that means starting mechanism in diesel position). The regular engine valve clearance is .017 inch. If you are getting into adjustments on the starting system, I strongly suggest you get a service manual. There are a number of things that need to be checked out to get it all right.