Loud Thud Feeling When Braking

I have a 1970 Dodge Dart Swinger with a slant six, that was converted from Manual Drum brakes all around, to Power Brakes with front discs and rear drums.

Just recently this Thud noise/feeling has been happening when I brake at low speeds. When I drive and come up to stop lights the car will drive fine, and when I brake coming up to a stoplight there is a noticeable THUD when I apply the brakes. There is no difference in power at all after it makes this noise. What could it be? The A904 transmission was not rebuilt, and I put in a new transmission mount and new U-joints with new spider gears in my 7.25'' differential.

Thank you for any ideas or possibilities for this issue, I don't like it and I'd like to fix it before something breaks while I'm driving.