Now look at above picture. (I know I'm no artist) Click on it, it gets larger to see better.

What will happen when you cut the top of the head tube on the dual clamp type and have to leave the top clamp where it sits? Frame and lower clamp go up and thus change head angle.
The Lefty fork will work on this same principal

What will happen on a single clamp (or crown) equipped bike when you cut the bottom of the head tube and have to leave the lower clamp where it sits? Frame goes down and sharpens head angle.

It's really not that hard to fathom.
Other people really can get things right even when you can not, PETE.

I've done exactly this a few years ago on a 98 FSR frame that we put a 7" Risse Trixxxy fork and 3" tires on. We had to to make the clearance for the larger tires as it would BARELY accept a 2.35", and so I did the cutting with the fore mentioned facing tool (look back on page 1 where I said to use this)
The Kevin Risse rear linkage and shock choice gave us 6.5" in the rear and I cut most of the materiel off the top of the head tube (3/8" to be exact) and was able to get almost a half degree more slack at the head tube because of this.

Now go attack someone who won't take the time to prove their point.