If it has the aluminum case with the emergency brake on the tailshaft, that's a one year only deal so if you want to keep that E-brake you need to just rebuild that trans. There's no witchcraft involved because it's a 727 so don't let anybody give you a bunch of crap about special antique parts made of $$$ unobtanium.
If you want to run a highstall converter, have the front pump and input shaft updated to the later style, 67 and up IIRC. Lots of converter choices and cheaper than the few that are available for the 19 spline input.
Imperial Services used to sell a kit to adapt a pushbutton shifter to the lever style trans but I don't think it's available anymore. They're the same company that repops the shift cables.
Kevin