Don't forget that the fuel tanks are going to be different. And have to agree with chick, it will be expensive, both converting and insuring. And you can get MINT 3i's out the yellow paper for about 500quid. If you are intent on going ahead, best get the donor car, and your car side by side and swap bits one at a time.

I'm in the middle of doing a conversion to my burds old 1.1 fez, so have a look at my post from time to time.