6 lug bolt pattern/center hub question

[DA68]

I've been all over the forum and other sites and can't get an answer on this question. I've been on the hunt for some 15" clip style wheels for my '59 GMC 100. I found a local guy who has some 6 lug wheels from a '48 Chevy car (did they put 6 lugs on cars?). He claims the bolt pattern is 5.5" and the center bore is 3.75". I measured my wheels and come up with 5.5" and 3.5 center bore. Can anyone tell me if the 6 lug cars and trucks used the same wheels? Thank you in advance for any help.