The 4T60 came out in 91-92 model year. It was slighly improved over the 440-T4, mainly because it got all the latest design updates from GM that fixed a lot of problems that the earlier 440-T4's had.
The OD (440-T4, 4T60) trans will require a different inner tripod on the driver's side than what works with the 125-C. Also, the OD trans is bigger on the driver's side as well and will require a shorter axle on that side. I have found that a left CV axle from a 94 Beretta with a 3100/4T60-E works well in most applications. I also believe the stock left CV axle from the Fiero works on the pass side of the 4T60. I don't know for sure as I had done this with a 4T60-E and can't remember if there is a slight length difference. I think there may be. In any event I think the right 94 beretta axle will also work here too.
The 440-T4 mounts that fit a 91 LeSabre will work in the Fiero however the slots in the Fiero cradle will need to be enlongated to allow the trans to slide slightly to the driver's side. This is to compensate for the extra length of the trans.
Other than that, check out: http://spacecoastfieros.com/tech/440-4T60/ on how to modify the stock fiero shift cable bracket to work with the OD trans.
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