I've just bought one and Racing Green told me that one loom fits both 4.2 and 4.5 V8 cars.

Let me know how you get on fitting it as mine is still to do

Message M3 John. I think that is where you are getting that from. I can't remember the outcome.

I can't remember if they're the same or not .. however the 4.5 pots rotate the opposite direction to the 4.2 pots, so if the wiring was the same and the throttle pot resistance direction the same you'd get one of the versions starting at throttle site 15 and going down to site 1 as you open the throttle
if it does turn out that way all you need to do is swap the outer wires on each throttle pot plug over, that puts the +5v input and earth the other way around so reversing the signal output, but for the life of me i can't remember if you would have to or not. try it and see? You won't blow anything up in the ecu, you'd just get a reversed throttle site versus throttle position if they're wrongly wired.

Looms are the same.
The only difference is at the connector to the throttle pots. Basically you just need to swap around two wires at the t-pot plug (leave the earth alone) and everything will be fine. (I can't remember the direction of rotation but...) The throttle pots on the 4.2 operate the opposite way to the 4.5. If you get them wrong the ECU doesn't know what on earth is going on as it sees the throttles at mechanical zero but electrically it's seeing maximum throttle !