How do the plug readings look and have you had to stagger jet the carburetor to clean it up? My first line of thought is that the runner dividers need trimmed back 90 degrees from the carburetor flange to the port floor. Look at the odd inverted triangular window from the center of the plenum to the outside runners. I may be wrong, but to me it just doesn't look like a happy flow path for the mixture to make it's way out of the plenum to the inside cylinders. Think about it plowing into the floor and then having to back up to make the turn around the runner divider.