Either the RS or WRX shifter would work (KB-001 or KB-002). 002 is wider at the pivot. I don't understand how that would fit in the linkage rod though even after cutting it down. Better would be to swap to the newer parts so it bolts in without cutting anything.
What you would need to do that are the linkage rod and u-joint from a newer model. Like anything from say a 96-01 Impreza or 96-04 Legacy. I suppose you could use the wider wrx parts from 02-07 as well but it would probably be easiest to stick with "1st gen" stuff.
Also, worn bushings in the u-joint linkage are the cause for most shifter slop, it will make a bigger difference than a short throw lever at this point.
Easiest and cheapest thing would be to find a car being parted out and buy the whole shifter assembly.
The rod that should work and be available is 35041FA050
A new linkage to go with it is 35047AC030
You might need some additional hardware, a new roll pin (633046020), and a centering spring (33022GA081) as well.
New from subarugenuineparts that's about $100. You also need a newer shifter to go with it, the kartboy is another $100. I sell them and the urethane shifter bushings.
I have fresh linkage bushings, the kartboy short shifter, and kartboy front and rear shifter bushings, and it is amazing.
There's a diagram here, I will have to update that page with some part numbers and more info:
https://hellafunctional.com/?p=1626