LOL yeah OK... Did you verify the original engine by checking VIN??? Any vehicle as bad as you say probably wasn't orig by a long shot, if info could be found my bet it was orig a 390 car... The 302 '69 wagon I mentioned had a 390, C6 and small rear in it when it left my house, was only six years old(302 had a noise that turned out to be loose wrist pin, gouged block) ... I had to buy the frame mounts and have the drive shaft shortened to mate to the trans... Later I saw it at a hole in the wall car lot over in Norfolk so I stopped in to see what they wanted... He began to tell me how good of car it was and had had this big Mercury engine in it... I laughed and told him those valve covers were from a junked '67 410, engine was a '66 390 from a LTD... He wouldn't talk to me after that(yeah I knew it was a POS)...
This was thirty years ago, but I doubt this car had ever had an engine swap done on it. At that point in time there were just too many junkyards around with hundreds of engines to choose from if one needed another engine, this was a 4 dr car owned by a dirt poor family that lived in a run down shack in the middle of a bean field.
I don't know about all that. My Dad bought a brand new 1967 Custom 500 in Oct, 1966. It came with a 289, automatic, and 9" rear end. I know for a fact it was never altered. I read the tag on the rear end, I can't remember what ratio it had now, too many years ago. I put brakes on it and had to get the axle tag numbers to get the proper brake shoes, so I know it was a 9".