I've been a falcon man for years, love the old grumbly beasts...
But even I'm the first to admit, those 3.9's were CRAP!! Fine as a car, but like mudchuka said, just dont tow with them.......... and as that's pretty much what you do with a falcon, (tow), they lost alot of friends over that dog of a motor.... known for oil leaks too
I was warned off them by my mate (a ford trained mechanic!!), but the 4.0 sorted all the problems out

so if it is a 4.0, go for it dude!!! They're around (depending on year) 180-210HP, and 330-350nM of torque, so they pull like horney school boys......
Go for it, you can do it Nickey!!! Kick him in the balls!!!!!
Oh yeah, as for pepping them.... I've seen a STOCK (yes, stock) XE 4.1 with a garret T66 and intercooler putting out 348KW!!!!!!! (I'll find the vid and try post it

)