It's very important to soak the friction plates in friction modifier for at least 15 minutes, but 2 hours would be better.
There's a special tool for checking to see what size shims to use, but you can probably just put in the biggest shims and try to rotate them into the carrier housing. If they don't go, drop to the next biggest size. Mine went in with the biggest shims.
Gear oil really stinks.
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351W 89 Mustang GT Convertible