51 shoebox rear end

i have this 51 custom and it has been built several times and modified a lot. Does anybody out there know how to tell if the rear end under it now is original or if it has been swapped? its been clipped and has a 302 and AOD in it so im betting its not the original but im not sure how to tell. sorry to bog down the board with my newbie question but i got to start somewhere right?

 

If it's got a cover on the back side it's not original...a photo would help us here.

 

Ok here is a pic, I also just noticed that it looks as if it has been shortened on both sides, welded seems I never noticed.

 

thats not original looks like an 8 inch maybe a9 inch

 

8" Ford...