That would be, IMO, inadequate gripping power by the tires combined with inadequate rear brake pressure to equalize the force. '74 came with the 10.87 set up, which is not too bad a street set up. I'd also bet you have 10" drums on the rear.
First off I'd check and verify that your rears are correctly adjusted, are getting sufficient pressure to expand tha shoes, and that all the hardware internal to the drums is in good shape.
The biggest benefit of discs, front or rear, is not that they provide greater braking force than drums, but rather that they do not fade as quickly when subjected to repeated hard braking situations.