I plan to have my gunsmith install a USA made slant break on my Hungarian SA85M.

Do I need to use a left or right hand thread on it to get it to line up properly?

14x1mm LH

You'll have to also have him install a 'muzzle device indexing pin' which is a plunger and spring that fits in the hole in the front of the sight base. The front sight base pin is removed, the spring and plunger goes in and the pin is replaced.
The slant brake (and all others) have notches in them so the index pin snaps into the notch and assures the device is indexed correctly plus it prevents it from unthreading.