Yes cutting your heads by .040 will reduce the current clearence from .120 to .080 safe enough.

But, there is more then just the Depth to worry about. Theres the Radial diameter clearence to consider too.

I do not know what diameter the 6 pac pistons are cut for, Stock, 2.14 The depth clearence at .080 would be fine IMO

But you also need to check for radial clearence at the edge of the valve. I had that problem on my stock fly cut pistons. I had to reclearence the radial clearence after noticing valve contact kiss marks.

I know these things because I am a Keyboard Racer. As I havent raced at the track in 4 years. mike