I just changed my 66 over to a manual( no power) 4 wheel drum dual master cylinder a few weeks ago. Running the lines was the toughest part.
Go to Partsource ( if you have one) or Canadian Tire and ask for a master cylinder from a 68 1/2 ton chevy. ( Impala etc is the same one). It bolts right up exactly the way your old one did. I just had to grind the shoulders of the bolts slightly. Use the same rod from the pedal- you don't even have to adjust it.
I can't find my receipt for it to give you the part number but I'm sure it was only about 35.00
