Late shift in all gears or one/some?
All can be crap in the governor. Can DIY but be very careful or can damage springs etc. Gov in part of VSS gearing.
If the trans was made for 4 cyl or another car heavier/lighter then Fiero may need governor weight/spring changes.
If you mod the TV cable to fit a different engine, very likely need pressure gauge on the trans to tell TV is adjusted right.
Car will have problems shifting w/ either or bolt parts have problems.
Some/one is other problems in valve body etc. and is a big job to fix parts inside. Worse, Many versions of TH125c/3T40 and many parts depend year of trans make.
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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)
The Ogre's Fiero Cave