Impreza Group-N project.

[mc68]

as said above, it depends if its 4wd or fwd.

if you are planning to put an sti transmission into an NA impreza (you didnt mention if you are changing the engine with it)

The diferences would be;
Cable clutch
Fuel tank (if 2wd)
Rear subframe, upwrights and struts (if 2wd)
The NA 4wd's had a tiny rear diff, but the R160/180 still fits in the rear cradle
tailshaft to suit the diff you are putting in (i assume R160, but GC RA's came with R180's sooo?)
the drive shafts will need to be changed to suit the front and rear diff (the front hubs will take most drive shafts of the gc era -they didn't change-, but the rears will be very diferent, and require the hubs to suit the driveshafts that suit the diff you are putting in)

with a subaru everything bolts in, its just a matter of having the right bits to do it

and dinges, ive put many 6 speeds in gc8's, and they fit in the tunnel with no mods at all, got a pic of the bulges you speak of?