Most large motor fords past about 1970(I think ) had the c6 transmission behind them. As such, you should be able to either locate a decent core and rebuild it yourself or buy a rebuilt one for your T-bird. As for the C4, it seems to me that there are companies offering a race-built c4 for mustang applications. I Dont know if this helps, but you may be willing to spend the money for a pre-built transmission