Drive line /9 inch help

You might need a puller to get the pinion yoke off after removing the nut. LAst one I did had to be removed that way. Don't sweat the seal, that'd be a good time to change it anyway. All you really need anyway is the old U joint to get the right size for the new shaft. Or just measure across the yoke from side to side, inside the ears in the "U's" and the "U" radius.

Don't take that yoke off the rear end unless you are prepared to buy a new crush sleeve and know how to set it up properly with an inch pounds torque wrench etc. If you take that nut off, your chances of putting in back in with the proper bearing preload is basically zero, and you'll have a noisy rear end that will drive you nuts.
You should be able to measure the characteristics of the yoke and the driveshaft place should know which u-joint they need to use. Usually they will ask you for a measurement from the back of the cups in the pinion yoke to the front of the tranny yoke cups when the tranny yoke is like an 1-1/2" from bottoming out. But that's just one way, and make sure to go by however they like to measure them up.
But save yourself the headache of having to bother with the pinion preload,

I just swapped the yoke on the 9" in my 77 Comet and have had no problems from retorquing the nut. Done this on 8.8's too.

D.Hearne said:

I just swapped the yoke on the 9" in my 77 Comet and have had no problems from retorquing the nut. Done this on 8.8's too.

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I hear what you're saying, but you getting away with it doesn't mean he will get away it, or that you will the next time.
I'd rather give him the tech, like is shown here http://www.ford-trucks.com/article/idx/0/065/article/Ford_9_Inch_RebuildGear_Setup.html
and then let him choose to abandon it if he wishes to.