Measure the height of the Mustang spindle and compare that to the Falcon. The PRE 1967 Mustang and Falcon have the same spindle height. I remember that Ford changed the Mustang disc brakes between the 67 and 68 model year and the 68 spindle is taller then the eraly Falcon but have no info on the 67 spindle.
Yes, I have heard of many differnent spindles being attached to Falcons, like the guy on the old board that claimed the Pinto spindles "fit" the Falcon ball joints. But was the geometry of the donor spindle the same as the early Falcon?
In the case of the Pinto set up, NO. The only time the geometry was correct was when the wheels were pointed straight forward.
So, as in this case where someone states his father did a swap with 68 spindles, if that were possible everyone would be adding 68 and 69 Mustangs spindles to early falcons, WHICH THEY ARE NOT!
Like i said, they may fit but is the geometry correct.
Now that you mention it I do remember someone mentioning Mustang spindles on a Falcon drag car because the geometry set the wheels more vertical therefore less drag. But drag cars don't have to go around corners as often as street driven cars sop that geometry would not be for street use.