[Sirrotica]

Don't forget the 75 400 has a bevel around the outside edge of the crown of the piston to further lower the compression, I still would probably go with the 400 than the 389, the compression with the 6X heads on either block will probably be very similar considering the 389 doesn't have the bevel on the crown of the piston.

The 6X heads will breathe better than the 91 heads all around, and as has been said don't discount how well they can run with stock compression. 400 rods are also marginally stronger than 389 rods, probably not going to make a difference in this application though.

I would not put any money into the 389 just to sell it, get the date casting codes and post the parts with block code and casting dates and ask whatever basic comparable 390 ford parts would bring from a full size galaxie, or best offer. You might want to post the block codes and find out what body the engine came out of to find out what it's worth. If it's a GTO engine it might bring more money. The first question you're going to get is block code and casting date from most people on here. 65 389s have thin castings and many times will only go .030 over.

Just checked the 91 cylinder head code and it shows 1966 389 2 bbl 8.6 to 1 compression. That may be what year the 389 if the block matches the heads. Most likely came in a 2bbl Catalina/Ventura. Also a thin wall block many times too, .030 overbore if you don't sonic check it, sometimes they are thicker, no rhyme nor reason to wall thickness.