Only runs 40 Degrees advanced

It wouldn’t be a rich mixture that would cause this issue, it would be a very lean mixture. Much like at cruise rpm when the cylinder charge gets diluted by exhaust and the carburetor is just off idle and it eats up to 52 degrees of mechanical and vacuum advance. But, if it were this lean, it would have a tendency to overheat. It sounds more like a vacuum leak or more likely that changing the sniper to not control the distributor has caused an unseen change in the fuel delivery curve. I would like to hear more specifics of the build, though. Displacement, camshaft type, static compression, etc.