Might also be the pads are poor material and glazed. "The basics." Rear drums: Make sure of hte usual, good parts, not all rusted/ bound up, adjusted, good shoe material AND IF THE drums have been turned out too much, they are now LARGER and without "arcing" the shoes to fit, only contact the shoe "middle" of the lining. This does two things: Because it's working on a fraction of the shoe surface, it overheats and glazes the lining, and because there is no surface, it just doesn't have much stopping force.
Also I would not get too excited about slotted rotors. Get some good pads if not, and have the factory ones turned.
My 67 when I had it together, I "threw it together" (to see if it would work, with the factory drum master. Worked find. I had 73/ 74 front disc, and Ford Versailles rear disk. You did have to press it some, but you could lock it up or nearly so at freeway speed. It REALLY stopped.
