Mate go the 4.2 you won't regret it. There is a big deifference between a 3l and a 4.2.
You can get alot of power out of a 4.2 if you spend a bit of money.
Fity as for getting the tacho to pick up I believe you need to get a whole new dash cluster.
I know the 2.8 one still works but the 3l ones are different.
You will also need to change your throttle cable over as it will be computerized and 4.2 is just a
Straight cable.