Just have the parts store look up the origional stud for your truck and look at the specs for it. Then get one how ever much longer you need.
O'Reilly's web site lists the specs. This is for a 74 K10 with a 350:
http://www.oreillyauto.com/site/c/de...2990&ppt=C0337
Just select your truck search for wheel stud. And BTW a decent parts store should be able to do this for you, but I know those are becoming few and far between.