My understanding is that those conversion necks are designed to allow a Fender Strat or Tele to accommodate a different scale length neck.
On a Fender neck, the fretboard extends beyond the heel of the neck, leaving a little lip of fretboard over the body. Fender guitars have their bridge measured an exact distance from the body's heel pocket in anticipation of this overlap. G&L guitar necks do not have any frets beyond the heel, so the distance between the heel of the neck and the bridge is different.
If I wanted to play two octaves on my Fender Strat, I could install one of these conversion necks and even though I have a longer neck with 3 more frets on it. it will still intone properly because the scale length will be calculated using the (known) distance between the bridge and heel pocket of my Strat. But since that measurement is different on a G&L guitar, even if the neck were shaved to fit the G&L pocket - it would never intone properly because it assumes a different measurement between the heel and bridge.
That how it looks to me at least. Unless things have changed at Warmoth, the conversion necks only convert guitars that use Fender's distance between heel and bridge. They depend upon (i.e. assume) that critical measurement between the heel and the bridge to calculate the fret placement on those necks for proper intonation.
Note, Warmoth's website still reads:
http://www.warmoth.com/Guitar/Necks/faq2.aspx wrote:Other Brands
The following brands are generally not compatible with Warmoth replacement bodies and necks:
Epiphone
G & L
Gibson and other neck-through guitars
Ibanez
Jackson
Music Man
Paul Reed Smith