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There are some special tools required to rebuild your transmission
. These mostly consist of seal protectors and some specific measuring tools as well as a flat bar. Now there are ways around it, but I don't suggest installing lip seals in the clutch drums with feeler gauges. However, many people have done it and it does work. But, per Ford instructions there are a few special tools that they recommend to do the job right.
Here is what I would suggest and your mechanic may be able to assist with this, Have him pull the transmission out, and take it to get it rebuilt if he can't do it. If you feel comfortable and trust him, then you may ask him to remove the transmission and then take it to the transmission shop for rebuilding. I have done that before in the past. and usually transmission shops will give a good deal to a fellow shop is they just bring them the unit and the transmission shop doesnt have to pull it out.
The first scenario is a consideration. Or, depending on what shape you find a used unit in, I would opt for getting a used unit, just be sure you get a warranty from a salvage yard if indeed you decide to do that. That's always tricky however, because you really don't know what kind of shape a used one is in, But if they will give you a 90 day warranty and it lasts that long, then it's most likely good to go for a while. Let me know if you have anymore questions.