sorry i didn't reply sooner, but i had tooth surgery [yanking out if you prefer ] and i have been kinda flat since. i'm slightly better now.
anyway, for reference, the wagner part number from my master catalog for your duster is : 506207
that fits from 1967-1970 with 10" drums, but the catalog doesn't list the bore size. however, i "think" it's 1". this is based on the 65-66 master used with 10" drums, manual or power, showing a 1" master being used.
remember, those years are a SINGLE POT master, so i could be [more than likely] wrong.
sure wouldn't be the first time, and won't be the last either !