Issues timing a new rebuilt 408

Are you using a vacuum advance and if so is it on ported or manifold vacuum? I would think you need to limit mechanical advance to about 30 degrees and hook the vacuum advance to manifold vacuum and have it limited to about 24-26 degrees. So at start up and idle you have the vacuum advance pull the advance all the way to 24-26 degrees and at cruise you have a fair amount of advance but at wide open throttle the vacuum advance drops off and the mechanical takes over and give you full advance. You will have to play with the spring tension on the mechanical weights. Good luck on your mission.