have you talked to Patricks? they're really good about talking over problems .... there's only one factory 3.90 rear in the TF years, and if that gearset is made/sold to replace the gears, and it can be installed in the carrier, it sure oughta fit under the cover!!
I'd give Patrick a call 1st thing today, if you're sure that rear is a 57, and you ordered a gearset for 57 ..... well, actually even if you're not sure
Bill
