I went to replace my front joint today and got it in about 1/4 of the way before I saw the same problem -- DUH!! It isn't just that the center of the U-joint is thicker, the big difference is the length of the axles the cup go on. Ford essentially shortened them and moved them outboard. I was going to try to put new cups on the old axles but they aren't at all the same.

So I'm off to NAPA for a freeze plug!

Edit: I got plenty of clearance by beating on the stock cup with a ball pein hammer.