I found this discussion at
Ford Muscle Forums : Ford Muscle Cars Tech Forum for installing a transmission. Apparently, all the talk about the converter being able to slide backward only applies to non-Ford converters that are bolted into place. Most Ford converters use studs and nuts, not bolts.
"On all the Ford automatics I've hooked up I lined up the studs with the holes as there never is enough room to spin the converter after it's bolted up. If the converter is seated properly into the transmission, when trans is bolted to the engine you should still be able to wiggle the converter back and forth slightly. I have never seen a studded converter spin behind the flex plate. Only converters that are held in place with bolts will do this. It has to be loose, so check to make sure it will move before tightening it, but you may only be able to feel the slop in the stud holes. IMHO, not one of Fords better ideas, but once you get it together, it's done."