Lugs, lugnuts, length of stud, safe?

have a 2 in. wheel spacer to mount on rear of older GMC. 2500. stock lugs are about 1 1/4 in length and lug nut tightens down but leaves about 5 rings of thread showing. .. how many full turns on a nut would be considered safe? (yea, I know, replace the stock lugs, not an option) would lock tight help ?