trying to tune my new /6 engine

Here is a great series of Holley tuning videos. A 390 should not be rich enough on even a stock slant to warrant cracking the secondaries. That's a pretty drastic step considering all the other adjustments those carburetors have. It's usually used to raise the vacuum signal on engines with really big camshafts. I'd be surprised if you really need to go there....but anything's possible I guess. Check out those videos. They should help if all is well with that carb. One thing I recommend different than the videos. Watch them first. You'll see that float adjustment is one of the last things they do. IMO, it should be the first. If the float adjustment is wrong, the very foundation for all the other adjustments will not be right. Good luck.

http://video.boxwrench.net/holley-carburetor.htm