What to do about a set of prefab doors?

[Streetmaster]

I recently purchased a set of prefab doors for my 76 GMC, The doors are new but there not GM doors. I stuck them on but they fit kinda funny, as if there twisted. I tried every thing i could think of to make them fit right, but no luck. When i get the top to fit some what close, the bottom sticks out to much. Im about to pull my hair out, because the doors cost $100. bucks apiece. So if anyone knows how to adjust a door to make it fit better, i'dd appreciate the info.