Just after a quick yes or no answer, will the cat back exhaust from my written off EF Mont sedan fit onto my EF Futura wagon??

you could make it fit

seriously, i think they fit up to a certain point, but if you look under your wagon, the exhaust pipes goes up and over the diff/axle housing on the right hand side. This is different to the sedans, but i cant remember how different. My ef wagon got written off so i cant have a look now, but assuming the ba wagon is the same, the muffler/silencer sits up by the right rear wheel tub, and the pipe drops down the back.

So at least your muffler/silencer/resonator will need to have modified piping if you want to mount it the same as stock.

Did this answer your question, i'm assuming you want to swap over the entire system?

From the head to the end of the center peice of exhaust is identical. The difference is only in the tailpipe.

Simple answer - no.
Fairmont = SWB
Wagon = LWB
No dice.

Agree

so by the sounds of it, asking for a quick simple answer was not the way to go. If it is only the tailpipe that is different, is it just the length, or the angle of bends, as I just came back from wreckers, looked at a few and they all look the same only the pipe is longer so it reaches the bumper.

If thats the case, then puttin a sedan tailpipe on a wagon would just mean that it wouldnt stickout to the same point as on a sedan? Correct way of thinking or not?

This is onyl half correct. The extractors, cat and mid section of the exhaust are identical regardless of weather it is a SWB sedan or a LWB sedan (i.e. fairlane/LTD) or wagon.
The tailpipe is the only difference - there are 2 different tailpipes - 1 for SWB sedan, 1 for LWB sedan (Fairlane/LTD) and 1 for wagons.
So all you'll need to do is go an get a new tailpipe or convert your old one into a diff dumper.