A lot depends on the era. I'll use my Skylark and it's GM A body siblings as a example. Not one external panel interchanges, and many internal ones won't either. Each divsion had their own V8s, the six was the Chev 250, except the Tempest, it's OHC six was in it's last year. Some transmissions were shared, some were division specific like Powerglide. The diffs will interchange, but there's internal differences. And other little things like Old's alloy Salisbury diff option, or Buick's alloy front drums. Things were different by the 1908 with some near identical GM offerings.