I don't mean to add fuel to the fire, but hopefully this will help.
1. You will get there, be patient.
2. There is a good chance the intake is not sealed, underneath, in the back (cyl #7 & #8)
3. How do you know it's running lean on #7/#8? Plug reading?

When I was fighting my 6-pack issues, neither of my center carb idle screws did anything. I had the transfer slots uncovered, go the center carb more closed & then only ONE of my center carb idle screws did anything. I took the carb apart, cleaned everything & found a "glob of oil smutz" inside the metering body...cleaned it & both idle screws now work. Hope that little story help... I know you have new carbs, but...