**** all that, just bolt the 4.6 in and try it, I reckon it'll be fine.
It'll bolt to your original 'box so I wouldn't worry, if it blows up then do something about it.
I've got a 3.5 V8 in a cortina on the original 2.0 axle, so far i've done 20,000 miles with it like this, why upgrade if...