The Ford and Mercury block are the same. It still came from Ford and will carry ford casting numbers from DIF (Dearborn Iron Foundry). Have him check the date code below the oil filter mount on the skirt of the block. There should be a date code that reveals year, month and day the block was cast. It might also tell you what shift the block was cast at the foundry. This will tell you for sure. From the tag it sounds like a 66 390 block. I'm sure it will all sort out fine. G.