hi guys just wondering if a 260 boss intake plenum will fit on a fairmont 220 3v 5.4 v8 cheers

No.

why not essentially the same engine arnt they

No, Heads are different.

yeah ok cheers

I've wondered that for awhile myself.
Having done some other research though, you can look for intakes to suit the 3v 4.6L Mustang on American go fast bits websites, as that is the same motor as our 3v 5.4L.
You can also look at aftermarket F150 (I think it was?) and also the Ford Triton trucks, circa the same year, look for the 3v 5.4L variants, I believe these intakes will work, same motor but different intake plenums.
Some of them have a front intake like you might find on the 5.4l GT motors, can change the throttle body to a larger one, or an oval shape, or twins.
(That would also mean you'd need to redesign your own airbox and intake piping to suit)

I have yet to see any 3v's in aus with the intake plenum changed, so this might be because its all too hard/expensive to try them and test for performance gain, or the platform is entirely ignored as a performance playground -atleast in this country, OR the intake that's on there flows fairly good already and you're unlikely to get any significant gain from changing it.

Intakes for the 4.6 shouldn't work on a 5.4. The 5.4 block is higher and wider.

Correct! 8.9" deck vs 10" deck.
You would need to widen a 4.6 Intake to fit a 5.4.