Is there a power brake booster/master cylinder that I can use to
replace my stock master cylinder? It's a 68 Dart sedan with 13" wheels
and 4 wheel drum brakes.

There is, but what's your goal?

I want to reduce the pedal pressure that I have to apply to the brake
pedal. I know the virtues of disc brakes but I prefer to keep the 4 wheel
drum setup. This car is used a a commuter car and does not see a
lot of hard driving that would lead to brake fade.

So SlantSixDan, what is the part? master cylinder and booster.
How will this effect my vacume pressure?

Here's one that'll work correctly for you. Hurry fast...