I have noticed a few threads now where napa wasn't of any help for studs. I just took one down to my local store (lucky for me it's a warehouse) and they mic-ed em and handed me 20 ready to go... Even had an inch longer as I asked for. Matching them up is much more productive than the alternative. IF you go by the 'book' then 50's 5/8's studs are the same.
What I have found in my run with these trucks is 9/16 went on 1.5 and 2 ton trucks 42' and earlier, and 46- received the 5/8 larger studs. Unconfirmed, but so far true of my herd and parts hunting.