[1996 328ti]

I know this has all been discussed.
A 3.45 is fine. You are going to rev almost 4K at 80 mph.
I'd install a diff made for a 6 cylinder.
I have twin cats that are fitted into a Y pipe to a single exhaust
I think it's a Dynamax or Dynaflow.
I had 323ti sport springs.

Replace the radiator and motormounts.
Will you have and CEL issues? I did.