Well I did do a disc swap on a 68 fury with 73 spindles.
I kept the drum booster and master cylinder.

It will stop the car. And under normal braking conditions it is fine.

BUT, I could never lock up the brakes so I know I did not have full braking potential.

You can get away with it, and you are probably already used to planning way ahead on stopping.
I picked up a 68 disc booster to try out, but got rid of the car before every trying it out to see if it made a difference, so I cannot say for sure if it matters.