You said you could see movement of the shift levers and you said it was a 55 or later Saginaw, so it's not a SM420 top loader. Is the shift lever stripped on the shaft that goes in the trans? You might try shifting the levers by hand. Are they stuck? The shifting fork might jave dent or slipped out of position. You'll probably have to disconnect the levers, drain the oil and remove the side cover to look around in there.
