Power brake drum/drum master cylinder

[1970Chevrolet]

I searched and searched and can't find an answer so hopefully someone can help me out. I need to find a power brake drum/drum master cylinder. I have a 1970 C10 and it has a power booster but it also has a manual master cylinder. so there is a lot of play in my pedal before it engages the brakes. It is my understanding that on a manual master cylinder, the hole in the cylinder for the push rod is like 1.5" deep and the hole on the power master cylinder is a very shallow indention. So my push rod is basically 1.5" away from touching the cylinder, which is causing so much play in my brake pedal. Does anyone know where I can find a power master cylinder? I have looked on all of the parts stores websites and can only find the manual ones. I'm not sure if there is another year that would interchange. Any help is appreciated, thanks.