I found a 1978 truck with a short bed and the guy is parting it out. I have been wanting to do the shortbed conversion for a while now and was wondering if a 78 box will line up with the 92 cab. By line up I mean same body lines

 

No, front fenders will be different style/lines than '78 bed. Lower body line not the same, panels have different shape. 81-93 would look right in my opinion.

 

Buy the front clip and doors too