Seeking insight on this front disc conversion kit

You could use your existing master cylinder, but you'd need to do minor surgery on it to get it to work....and then it would still have a small reservoir for the front brakes. Just get one for a 74 Valiant/Duster with manual front disc brakes. Try to buy a used proportioning valve if you can. The new reproduction ones don't have a very good reputation. @RAT ROD AL has parted a few cars that would have had the right one, if he didn't sell it with the disc brakes.