There are definitely sites, and DIYs out there for this kind of thing.
Basically, from the 325, you will need:
- Wiriing harness
- Fuel pump(s)
- Motronic engine box
Those are the absolute basics. If you want, you can swap the rear brakes onto your 318i, because they'll be discs and your originals are drums. Much better stopping power and resistance to fade. Of course, whichever car has the best interior, do as much swapping as possible. Drop in the OBC from the 325, they're a good tool, and a fun toy.
To finish off the project, I reccomend shaving your 318i badges, but not reinstalling the 325 ones. This will give your car a sharper rear look, and won't betray the car's original make, nor tip people off to the fact they're dealing with a six-cylinder. If you want to be punchy, a little stealth can't hurt.
I'm sure more people can give you more technical info. As always good luck, and keep us posted.