318i to 325i is dead easy, took me a day and a half (on an 89 touring

).
Its basically take all the 318i stuff out and refit with the 325i stuff. There is a thread in the engine conversion section about doing just this. You will need the engine, loom, ECU, gearbox, prop and diff from the 325i.
As yours is a touring, only the front springs will need changing for the 6cyl items, although Im still running the 4cyl springs and other than being a bit bouncy its ok, they are lowered 4cyl springs though.
Wiring should just be plug and play at the round plug (C101) up by the fusebox.
The only hassle is the 318i wont have the mounts for 325i header tank and the AFM mounts are in the wrong place. Some home made brackets will sort that.
This is just a quick reply, but I dont think Ive forgotten anything. Oh and if youre going to change to 6cyl, go for the 325i motor
