When it comes to 7-segment displays there are 2 types, "common anode" and "common cathode" both require a different way of driving them. One suggestion is dump the 74ls78 bcd to 7 segment decoder and 74ls138 decoder and go to a MAX7219 and an LTC-4727JR. The MAX7219 communicates with 3 wires. The LTC-4727JR is a very flexable display.

Best, Ed