My New 850 MK1

[Jared Mk3]

You'd be able to fit the 1275, but the gear selector would be different. Unless, you rebuild the diff housing to accept the earlier style gear selector. Another option might the 998 engine, especially if it's just suburban traffic. Choice is ultimately yours, but Mk1 cars are getting harder to find - so keep that in mind for future value if you ever sell.