Sorry for the late reply. I hope the OP has an alert for replies, or checks back soon.
I don't know those cars very well but, just going by general information, a 5 speed box is bigger than a 4 speed. I don't mean that in jest; some folk really don't realise the actual housing is larger.
Using my knowledge of Imprezas now;
My STi is a 5 speed. In about 2004-5 Subaru changed the STi to a 6 speed. I have seen many Scooby owners do a box swap. It requires a lot of work beyond just the box itself as the box is physically longer than the 5 speed. The diffs had to be worked on to.
Plus wiring loom work and ECU remap so it knows to look for more gears.
And, I suspect, a lot more that I can't recall right now.
Estimates for this work ran up to £10k.
Most things are possible with enough money thrown at it. Is it worth it? Only you can answer that.