There is a guy on classicbroncos.com that used long tube headers intended for early Mustang and from what I remember it worked out ok. I will see if I can find the post and provide a link. If they are priced real well I would be tempted to buy them, if they don't work you can probably sell them on fordsix.
It may depend on what kind of long tube headers you are considering. Some are 'tuned' I think is the term, the tubes are all the same length so the tubes kind of have extra bends- others all make the most direct path to the collecter so the tube length is not the same for all six if i am explaining that right.
