I dunno - that sounds scarey - cutting suspension compnents in half...
Plus, I assume you'd need both right hand and left hand threads (plus lock nuts) to make an adjustable upper A-arm. LH taps & dies are not easily (or cheaply) found.
Sounds like you should order the pre-made adjustable arms and know it has been done right.
There is a picture of the two types in reply #8 of
this thread. It doesn't look as easy as just cutting one in half.