Changing engines wont affect the revs at 100kmh. The final drive on a 2sps & 3spd box is still 1:1. Of course a 250 will be a lot more driveable due to increaesed power.
Id be looking at changing diff ratio. XP Fairmont had 3.2:1 but are hard to find. A 5 speed manual would be a better way to go.
As far as fitting a C4 or BW35 it will fit but you need to open up the tunnel entrance to fit. I have just done this with my XM, its not that hard.
250 will clear the bonnet but you need different mounts.
Gearshift can be column, mine is. Or floorshift but if you have front bench its a bit tight. Tailshaft on BW35 is the same length, not sure on C4.
Heres a pic that may help
