Holley Idle Screws Not Responsive

I have an XE268H cam in my 360 and it likes 17 degrees of intial advance, I would think you will need that or more. If your MP disributor was built after 2002 it will have an adjustable mechanical advance, just look below the plate the pick is mounted on for two button head torx screws to confirm if it's an adjustable one. You will need to limit the total timing to prevent to much on the top end. If you have the adjustable distributor or get one there is a Mallory kit #29014 that has a selection of springs for adjusting the rate, spacers to set the mechanical and charts to help you understand what the changes will do.

Remember the first step in tuning an engine is to get the timing right, then worry about the carb.