Hi, this is my first post. I usually get the info I need by reading previous posts but need help. I have a 383 stroker with a Mallory Unilite with vacuum advance. I unplugged the vacuum advance and set the total timing to 38 degrees at 3000 rpm. This caused the initial timing to be 16 degrees. When I plug the vacuum advance back in the initial timing jumped to 39 degrees. Is the vacuum advance can adjustable on these distributors? What should I set it to? How do I set it? I only intend to drag race this engine. Would mechanical advance only be better? Also what would be better, the vacuum port above or below the throttle blades? I have an Edelbrock 750 cfm on it.

Thanks in advance for any help.