I was thinking, could you use an XH ECU on a boxcar?? I was thinking of getting one mainly to control the thermo fans. Would there be much other benifit in using it, in terms of having a better tune for example?

The XH has 148kw and my 3.9 only has 139kw. Your thoughts??

Engine wise, there are a few 'iffies'. The ford XH shop manual says it runs a std EF/EL cam, so I'd say you should run the same to match the tune. The XH runs EF/EL compression ratios also. I'll confirm the cam in an XH when I do my BBM conversion. The XH also runs a knock sensor, so it would be wise to have one I would think - late ED blocks onwards have this provision - IIRC someone said an AU knock sensor will fit one of the engine block bolt holes of earlier blocks (chase this one up with one of the engine guys on here).
If you're running EF/EL climate control, you'll have issues with the XH ECU - you'll consistently get E6, E7 & E8 errors, and the CC will not cool as good as it should - if you feel the high pressure refrigerant lines, they'll get very hot. An EL ECU will fix the CC issues.
So all in all, I'd suggest running an EL ECU, with EL climate control (you were converting anyway ??) and run a BBM. along with your thermos - would be the best setup.

thanks mate, this is only for my stock 3.9 with the log manifold.

the car i am doing the conversion on is my 5.0L V8

I was just brain storming on an alternative on how to get my thermos running on my 3.9 becuase the clutch fan sounds crap. And mabey get the benifit of a better ECU as well. But may look like the XH unit is more trouble then its worth??

No probs dude - that's what I was assuming.

Well if you're running std A/C or EA-ED climate control, I doubt the XH unit would be a problem.