If you haven’t already done so, you will need to change out the proportioning valve. It is made for a rear drum set up and has a 10 lb residual valve in it to keep the rear brake shoes from backing away too much from the drum. Since you now have disc brakes in the rear the 10lb residual valve needs to be removed.
Here is a link.
http://flynbye.com/catalog/i67.html
For a master cylinder, you may need to upgrade to a master cylinder that has a larger reservoir for the rear disc brakes. Rear drum brakes require far less brake fluid capacity than rear disc brakes. Just remember that the rear reservoir is for the front brakes and the front reservoir is for the rear brakes. There are some 70s vette master cylinders that will bolt up to your vacuum booster and have the correct brake line outlets, but these are cast iron. If you want aluminum, you will have to go aftermarket. If using the vacuum power booster, you can go with a 1” bore master cylinder, if going manual, a 7/8” strait bore master cylinder is recommended.