Dominik,

There are quite a few things that can go wrong with the Fiero clutch.
Checking the cables is a good start! :) Also, get a magnet and put it
on the clutch pedal shaft (not the part that you step on). If it
sticks, good. That means you have a metal clutch pedal, and that is
good. If it falls off without sticking then you have an aluminum
pedal, and that is bad. The original pedal was aluminum and it bends
over time, and that causes the clutch to not release all the way. The
Fiero Store (www.fierostore.com) sells replacement steel pedals.

Another issue is bleeding the clutch, but it sounds like this is not
the problem.

Something else to look for is the arm on the outside of the
transmission (where the clutch slave pushes). This arm should be a
one-piece cast iron unit - NOT a stamped steel one with a plastic
piece that touches the clutch slave rod. These stamped steel pieces
collapse over time and need to be replaced.

The pressure plate fingers also fail, and that can cause the trans to
not shift. In that case you'd need a new clutch setup.

Lastly, yes - you can replace the 4 speed with the 5 speed transaxle.
It's a pretty straightforward swap, but a lot of labor involved.
You'll want to do a new clutch while it's out as that is the perfect
time. ;)

Eric