My experience is that air leaks are often overated, and blamed more than they should be, it's often a mechanical issue. If there is a lot of "mushroom" effect on your clutch shoes than dremel the edge off and start with fresh clutch bearings, and also double check linkage. Also make sure you have a slight bit of back and forth play on the clutch bell front to back on the bearings. The clutch bell shouldn't turn when you roll the flywheel back and forth slowly.

You can check the linkage without being at the track or even having the car running. If you hit the brake and the carb closes further, that's all you need to look for to see if that is it, takes 2 seconds.